Merge develop into feat/git-sync — unify converter on the branch (#293/#326 step 6a)

Per maintainer directive (#119 comment): land the canonical converter on the
git-sync branch so sync is tested on the real format, NOT a dead legacy copy.
#119 itself stays FROZEN (не вливается) — this only merges develop in.

Resolutions (all git-sync converter conflicts → develop; engine kept as-is):
- Dropped the branch's legacy `packages/git-sync/src/lib/*` converter — the
  converter now lives solely in `@docmost/prosemirror-markdown` (#293); the
  engine (pull/push/stabilize/index) only switches its imports to the package
  (no logic change, verified by diff).
- Removed the branch's orphaned converter tests + fixtures under
  `packages/git-sync/test/` (their coverage moved to the package's own test
  suite on develop); git-sync/test now holds engine tests only.
- .gitignore / Dockerfile / test.yml / AGENTS.md: unioned — build/ ignored for
  every package; Dockerfile COPYs both prosemirror-markdown/build (mcp+git-sync
  runtime) and git-sync/build (git-sync's runtime consumer lands on this branch);
  CI builds prosemirror-markdown before git-sync/mcp.
- pnpm-lock.yaml regenerated for the merged workspace.

Branch adaptations to canon (server-side tests only — converter untouched, per
the guardrail that converter fixes go to the package on develop, fixtures-first):
- git-sync-converter-gate.spec.ts: heading textAlign and image width/height now
  round-trip via the canon trailing-comment forms (#9 `<!--attrs {...}-->`, #4
  `<!--img {...}-->`) instead of the old HTML-tag forms — expectations flipped to
  the real canon output. RESIDUAL: canon #4 does not yet carry image `align`
  (documented as a known divergence; fix belongs in the package on develop).
- schema-attribute-contract.spec.ts: the schema mirror moved from
  `@docmost/git-sync/lib/docmost-schema` to `@docmost/prosemirror-markdown`;
  import + jest source-mapper updated.

Verified: prosemirror-markdown/git-sync/mcp build clean; git-sync corpus green;
server `tsc --noEmit` 0; gate + schema-attribute-contract specs 32/32.

apps/server/src/collaboration/collaboration.handler.ts

@@ -6,6 +6,7 @@ import {
   tiptapExtensions,
 } from './collaboration.util';
 import {
+  removeYjsMarkByAttribute,
   replaceYjsMarkedText,
   setYjsMark,
   updateYjsMarkAttribute,
@@ -82,6 +83,40 @@ export class CollaborationHandler {
           },
         );
       },
+      deleteCommentMark: async (
+        documentName: string,
+        payload: {
+          commentId: string;
+          user: User;
+        },
+      ) => {
+        const { commentId, user } = payload;
+        // Ephemeral suggestions (#329): when a suggestion-edit is dismissed or an
+        // applied one has no replies, the comment is hard-deleted and its inline
+        // anchor must vanish too. Mirror resolveCommentMark exactly, but instead
+        // of flipping the mark's `resolved` attribute we STRIP the `comment` mark
+        // entirely via removeYjsMarkByAttribute so no orphan highlight remains in
+        // the collaborative document.
+        //
+        // Routing this through collaboration.gateway's handleYjsEvent means the
+        // COLLAB_DISABLE_REDIS path invokes this handler directly (never a silent
+        // no-op) and a missing live instance is a hard error — the same guarantee
+        // applyCommentSuggestion/resolveCommentMark rely on.
+        await this.withYdocConnection(
+          hocuspocus,
+          documentName,
+          { user },
+          (doc) => {
+            const fragment = doc.getXmlFragment('default');
+            removeYjsMarkByAttribute(
+              fragment,
+              'comment',
+              'commentId',
+              commentId,
+            );
+          },
+        );
+      },
       applyCommentSuggestion: async (
         documentName: string,
         payload: {