fix: switch issue templates to YAML forms, remove broken markdown templates

Markdown-based issue templates with frontmatter were silently ignored
by GitHub (likely an empty-string assignees field or frontmatter
format issue). Switched to the newer YAML-based issue forms which use
a structured schema format that GitHub explicitly supports.

- bug_report.yml: structured form with required fields for description,
  reproduction steps, environment, screenshots
- feature_request.yml: form with problem/motivation, proposed solution,
  alternatives, mockups
- config.yml: unchanged, keeps blank_issues_enabled: false

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -0,0 +1,53 @@
+name: 🐛 Bug report
+description: Report a bug or unexpected behaviour
+title: '[bug] '
+labels: ['bug']
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to report a bug! Please fill in the details below.
+  - type: textarea
+    id: description
+    attributes:
+      label: Describe the bug
+      description: A clear and concise description of what the bug is.
+      placeholder: What happened? What did you expect to happen?
+    validations:
+      required: true
+  - type: textarea
+    id: reproduction
+    attributes:
+      label: To reproduce
+      description: Steps to reproduce the behaviour.
+      placeholder: |
+        1. Go to '...'
+        2. Tap on '...'
+        3. Scroll to '...'
+        4. See error
+    validations:
+      required: true
+  - type: textarea
+    id: screenshots
+    attributes:
+      label: Screenshots / screen recordings
+      description: If applicable, add screenshots or a screen recording to help explain the problem.
+      placeholder: Drag & drop images here
+  - type: textarea
+    id: environment
+    attributes:
+      label: Environment
+      description: Device, OS version, app version, and Hermes dashboard version.
+      placeholder: |
+        - Device: e.g. Pixel 8, Samsung Galaxy S24
+        - Android version: e.g. Android 14, Android 15
+        - App version: e.g. v1.2.3, or the commit SHA
+        - Hermes dashboard version: if known
+    validations:
+      required: true
+  - type: textarea
+    id: additional
+    attributes:
+      label: Additional context
+      description: Error logs, crash reports, adb logcat output, or anything else relevant.
+      placeholder: Remove any sensitive info (tokens, hostnames) before pasting.