feat(#533): make /fork work via chat (session.branch) (#565)

* feat(#533): add session.branch (fork a conversation) to mobile

The gateway already supports session.branch (tui_gateway/server.py), but
the mobile had no client surface. This adds:
- WsMethods.SESSION_BRANCH = "session.branch"
- A Branch (CallSplit) action on each session card that forks it via the
  session.branch WS RPC with the source session_id (and optional name).
- branchedSessionId state -> SessionsScreen navigates to the freshly
  forked session on success; RpcError -> toast; spinner while branching.
- SessionsViewModelTest (4 tests): forwards session_id, forwards name,
  sets branchedSessionId on success, toast+no-branch on RpcError, and
  consumeBranchedSession clears the pending id.

Verified against the live gateway contract: session.branch expects
{session_id, name?} and returns {session_id, title, parent}. Local
testDebugUnitTest: 4/4 pass, ktlint clean.

* refactor(#533): make /fork work via chat instead of a session-menu button

Per review feedback: the session.branch surface belongs in the chat
composer, not the session list menu.

- SlashCommandDispatcher: /fork and /branch -> new SlashResult.SessionBranch.
- ChatViewModel: handles SessionBranch by sending session.branch with the
  active runtime session id (optional arg = branch title), and on the RPC
  result switches the current session to the new forked one (mirrors the
  SESSION_CREATE handler). This fixes /fork, which previously fell through to
  command.dispatch and 4018'd (backend only knows session.branch).
- Removed the Branch (CallSplit) button + navigation LaunchedEffect from
  SessionsScreen/SessionsViewModel (reverted those two files to main) and
  dropped the now-unused sessions_action_branch string.
- Tests: deleted SessionsViewModelTest (branch-only); added /fork -> SessionBranch
  to SlashCommandDispatcherTest and two ChatViewModel tests asserting /fork
  sends session.branch with the session id + optional name.

Local testDebugUnitTest: SlashCommandDispatcherTest 8/8, ChatViewModelTest 50/50,
0 failures. ktlint clean.

---------

Co-authored-by: ronia <ronia@m57>

app/src/main/java/com/m57/hermescontrol/data/ws/WsMethods.kt

@@ -12,6 +12,7 @@ object WsMethods {
     const val SESSION_INTERRUPT = "session.interrupt"
     const val SESSION_DELETE = "session.delete"
     const val SESSION_TITLE = "session.title"
+    const val SESSION_BRANCH = "session.branch"
 
     // ── Interaction ───────────────────────────────────────────────────────
     const val PROMPT_SUBMIT = "prompt.submit"

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt

@@ -466,6 +466,25 @@ class ChatViewModel(
                 loadSessions()
             }
 
+            WsMethods.SESSION_BRANCH -> {
+                val resultMap = result as? Map<String, Any?> ?: return
+                val newId = resultMap["session_id"] as? String ?: return
+                runtimeSessionId = newId
+                _uiState.update {
+                    it.copy(
+                        currentSessionId = newId,
+                        isLoading = false,
+                        messages = emptyList(),
+                        chatTitle = (resultMap["title"] as? String)?.takeIf { t -> t.isNotBlank() } ?: "Hermes",
+                    )
+                }
+                ActiveSessionHolder.set(newId)
+                _streamingState.update { StreamingState() }
+                addSystemMessage("Session branched", persist = true)
+                loadSessionMessages(newId)
+                loadSessions()
+            }
+
             WsMethods.SESSION_LIST -> {
                 val resultMap = result as? Map<String, Any?> ?: return
                 val sessionsList = resultMap["sessions"] as? List<Map<String, Any?>> ?: return
@@ -790,12 +809,40 @@ class ChatViewModel(
                 createNewSession()
             }
 
+            is SlashResult.SessionBranch -> {
+                branchSession(command)
+            }
+
             is SlashResult.RpcDispatch -> {
                 dispatchViaRpc(command)
             }
         }
     }
 
+    /**
+     * Fork the active conversation via the session.branch WS RPC (issue #533).
+     * The backend already supports session.branch; the mobile previously had
+     * no client surface, so `/fork` fell through to command.dispatch and 4018'd.
+     * The optional arg becomes the new branch's title.
+     */
+    private fun branchSession(command: String) {
+        val sessionId = runtimeSessionId
+        if (sessionId == null) {
+            addAssistantMessage("No active session. Use `/new` to create one.")
+            return
+        }
+        val arg = command.split(" ", limit = 2).getOrElse(1) { "" }.trim()
+        val params = mutableMapOf<String, Any>("session_id" to sessionId)
+        if (arg.isNotBlank()) params["name"] = arg
+        viewModelScope.launch(Dispatchers.IO) {
+            wsClient.send(
+                WsMethods.SESSION_BRANCH,
+                params,
+                onSent = { id -> trackRequest(id, WsMethods.SESSION_BRANCH) },
+            )
+        }
+    }
+
     private fun dispatchViaRpc(command: String) {
         val sessionId = runtimeSessionId
         if (sessionId == null) {

app/src/main/java/com/m57/hermescontrol/ui/chat/SlashCommandDispatcher.kt

@@ -17,6 +17,7 @@ class SlashCommandDispatcher {
         return when (cmd) {
             "/stop", "/interrupt" -> SlashResult.Interrupt
             "/new" -> SlashResult.NewSession
+            "/fork", "/branch" -> SlashResult.SessionBranch
             else -> SlashResult.RpcDispatch
         }
     }
@@ -34,4 +35,7 @@ sealed class SlashResult {
 
     /** Forward to command.dispatch via WebSocket. */
     data object RpcDispatch : SlashResult()
+
+    /** Fork the active conversation via the session.branch WebSocket RPC. */
+    data object SessionBranch : SlashResult()
 }