Contribute rewrite Phase 1: CM6 markdown source editor

Swap the Contribute-mode editing surface from Tiptap WYSIWYG to a
CodeMirror 6 markdown source editor. Discuss mode and any read-only
viewing continues to render through Tiptap.

The §8.11 manual-edit debounce now reads the raw markdown source via
CodeMirror's doc, eliminating the lossy `editor.getText()` round-trip
that RFCView.jsx flagged as a §19.2 candidate; what the contributor
typed is exactly what gets POSTed to manual flush.

The §8.10 paragraph-margin gutter accent on Tiptap is dropped — it was
dead in read-only Discuss anyway, and Phase 4 of the Contribute
rewrite adds a change-anchored gutter against the CM6 raw pane.

The Tiptap-side accept-time injection of <span class="tracked-*">
markup also drops out — CM6 can't render those spans, and Phase 3's
preview pane is the proper home for tracked changes. The reviewMode /
DiffView toggle still works against marked-rendered HTML in the
interim until Phase 7 retires it.

Notes:
- Bundle gzipped grows ~50KB for CM6 modules (state/view/commands/
  language/lang-markdown). Mermaid in Phase 2 will be the larger lift
  and should lazy-load.
- Verified the CM6 editor mounts cleanly via Vite dev preview: ref
  handle (`view/getDoc/setDoc`) is wired, `onUpdate` fires on doc
  changes, gutter / line-numbers / active-line all render, and the
  source round-trips verbatim. Could not drive the full RFCView
  Contribute flow end-to-end without a running backend; the manual
  countdown + save-now + accept/decline pathways are verified by
  inspection only.
- 125 backend integration tests still green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -9,6 +9,11 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@codemirror/commands": "^6.10.3",
+    "@codemirror/lang-markdown": "^6.5.0",
+    "@codemirror/language": "^6.12.3",
+    "@codemirror/state": "^6.6.0",
+    "@codemirror/view": "^6.43.0",
     "@tiptap/extension-placeholder": "^3.5.0",
     "@tiptap/pm": "^3.5.0",
     "@tiptap/react": "^3.5.0",