feat(sessions): empty-session cleanup + latest-descendant resume + fresh-session 404 fix (#787, #840) (#841)

* feat(sessions): empty-session cleanup button + latest-descendant resume + fresh-session 404 fix (issues #787, #840)

- Sessions top bar: Delete-empty icon with count badge, grey/disabled when 0
  (GET /api/sessions/empty/count + DELETE /api/sessions/empty, confirm dialog)
- Resume flow resolves GET /api/sessions/{id}/latest-descendant first —
  branched sessions continue in the newest child leaf, not the stale parent
  (best-effort; falls back to the requested id)
- syncCurrentSession guarded by sessionHasServerPresence — a freshly created
  (never prompted) session has no server row yet; the REST transcript 404s
  and the resume retry spun forever (logcat 404 evidence)

* fix(sessions): clamp message-count fallback to backend limit cap (422)

fetchServerMessageCount requested limit=500 but the backend caps
session list page size at 100 (sessions.py Query le=100) — every
fallback scan 422'd (seen in device logcat after a branch). Sessions
are ordered recent, so the target is always in the top page.

* test(slash): make dispatch-capture assertions order-independent (CI flake)

The capture-race flake (SlashCommandDispatchRpcTest, hit 3x across CI and
local runs 2026-08-07): tests asserted on methodSlot.captured — the LAST
request captured — but the create flow's session.usage/context_breakdown
requests can land after the slash-dispatch one depending on suite order
(the class's own comment documents this cross-wiring). Converted the
three capture-based tests to capture ALL requests and find the target
record by method, with dispatch-before-fallback ordering for the 4018
test. Same assertions, no dependence on request interleaving.

* test(slash): convert remaining capture-race assertions (flake round 2)

The capture race moved to the focus/init tests (:424) after the first
three were fixed — every request-capture test in the class used the
same last-captured assumption. Converted the remaining five (init,
init+arg, focus, focus on, type=send) to the find-record-by-method
pattern; the type=send answers block reads methodCalls.last() since the
capture precedes the answer. Zero request() slot captures remain.

* test(slash): blocklist assertion counts command RPCs only (flake round 3)

The blocklisted-command test counted ALL request() calls and asserted
zero — the create flow's session.usage/context_breakdown noise can land
in the window after the stub registers (the same capture race that hit
the dispatch tests). Asserted instead that no COMMAND_DISPATCH or
SLASH_EXEC fires, which is the actual blocklist contract. E2E's static
Dispatchers mock stays (E2E VMs need IO routed to the test dispatcher —
removing it broke 20 E2E tests).

---------

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/ui/sessions/SessionsScreen.kt

@@ -43,6 +43,8 @@ import androidx.compose.material.icons.filled.Search
 import androidx.compose.material.icons.filled.SelectAll
 import androidx.compose.material.icons.filled.Terminal
 import androidx.compose.material3.AlertDialog
+import androidx.compose.material3.Badge
+import androidx.compose.material3.BadgedBox
 import androidx.compose.material3.Button
 import androidx.compose.material3.ButtonDefaults
 import androidx.compose.material3.Card
@@ -331,6 +333,29 @@ fun SessionsScreen(
         onClearToast = { viewModel.clearToast() },
     )
 
+    // Empty-session cleanup dialog (issue #787)
+    if (state.showEmptyCleanupDialog) {
+        AlertDialog(
+            onDismissRequest = { viewModel.hideEmptyCleanupDialog() },
+            title = { Text(stringResource(R.string.sessions_empty_cleanup_title)) },
+            text = { Text(stringResource(R.string.sessions_empty_cleanup_message, state.emptyCount)) },
+            confirmButton = {
+                TextButton(onClick = { viewModel.confirmEmptyCleanup() }) {
+                    if (state.isCleaningEmpty) {
+                        CircularProgressIndicator(modifier = Modifier.size(18.dp), strokeWidth = 2.dp)
+                    } else {
+                        Text(stringResource(R.string.sessions_empty_cleanup_confirm))
+                    }
+                }
+            },
+            dismissButton = {
+                TextButton(onClick = { viewModel.hideEmptyCleanupDialog() }) {
+                    Text(stringResource(R.string.action_cancel))
+                }
+            },
+        )
+    }
+
     // Prune dialog
     if (state.showPruneDialog) {
         AlertDialog(
@@ -517,6 +542,25 @@ fun SessionsScreen(
                             },
                     )
                 }
+                Spacer(modifier = Modifier.width(spacing.sm))
+                // Empty-session cleanup (issue #787) — grey/disabled when 0.
+                IconButton(
+                    onClick = { viewModel.requestEmptyCleanup() },
+                    enabled = state.emptyCount > 0,
+                ) {
+                    BadgedBox(
+                        badge = {
+                            if (state.emptyCount > 0) {
+                                Badge { Text("${state.emptyCount}") }
+                            }
+                        },
+                    ) {
+                        Icon(
+                            imageVector = Icons.Filled.DeleteSweep,
+                            contentDescription = stringResource(R.string.sessions_empty_cleanup_desc),
+                        )
+                    }
+                }
             }
 
             // List/state area takes all remaining height below the search bar.

app/src/main/java/com/m57/hermescontrol/ui/sessions/SessionsViewModel.kt

@@ -43,6 +43,11 @@ data class SessionsUiState(
     val deletingSessionIds: Set<String> = emptySet(),
     val showPruneDialog: Boolean = false,
     val isPruning: Boolean = false,
+    // Empty-session cleanup (issue #787): count of empty, ended, non-archived
+    // sessions — button is disabled/grey when 0.
+    val emptyCount: Int = 0,
+    val showEmptyCleanupDialog: Boolean = false,
+    val isCleaningEmpty: Boolean = false,
     val isDeletingBulk: Boolean = false,
     val toastMessage: String? = null,
     val sessionToDeleteConfirm: String? = null,
@@ -98,6 +103,7 @@ class SessionsViewModel :
 
     /** Load (or reload) sessions from page 0. Used by pull-to-refresh and initial load. */
     fun loadSessions() {
+        loadEmptyCount()
         loadJob =
             safeLaunchLoad(
                 currentJob = loadJob,
@@ -496,6 +502,68 @@ class SessionsViewModel :
         }
     }
 
+    // ── Empty-session cleanup (issue #787) ───────────────────────────────
+
+    /** Refresh the empty-session count (REST; auxiliary — failures are silent). */
+    fun loadEmptyCount() {
+        viewModelScope.launch {
+            val result =
+                runCatching {
+                    safeApiCall { ApiClient.hermesApi.getEmptySessionCount() }
+                }.getOrNull()
+            val count = (result as? NetworkResult.Success)?.data?.count
+            if (count != null) {
+                _uiState.update { it.copy(emptyCount = count) }
+            }
+        }
+    }
+
+    fun requestEmptyCleanup() {
+        _uiState.update { it.copy(showEmptyCleanupDialog = true) }
+    }
+
+    fun hideEmptyCleanupDialog() {
+        _uiState.update { it.copy(showEmptyCleanupDialog = false) }
+    }
+
+    fun confirmEmptyCleanup() {
+        _uiState.update { it.copy(isCleaningEmpty = true, showEmptyCleanupDialog = false) }
+        viewModelScope.launch {
+            val result =
+                runCatching {
+                    safeApiCall { ApiClient.hermesApi.deleteEmptySessions() }
+                }.getOrNull()
+            when (result) {
+                is NetworkResult.Success -> {
+                    val deleted = result.data.deleted
+                    _uiState.update {
+                        it.copy(
+                            isCleaningEmpty = false,
+                            emptyCount = 0,
+                            toastMessage =
+                                if (deleted > 0) {
+                                    "Deleted $deleted empty sessions"
+                                } else {
+                                    "No empty sessions to delete"
+                                },
+                        )
+                    }
+                    loadSessions()
+                }
+
+                else -> {
+                    val message = (result as? NetworkResult.Failure)?.error?.message ?: "Unknown error"
+                    _uiState.update {
+                        it.copy(
+                            isCleaningEmpty = false,
+                            toastMessage = "Cleanup failed: $message",
+                        )
+                    }
+                }
+            }
+        }
+    }
+
     // ── Toast ────────────────────────────────────────────────────────────
 
     override fun clearToast() {