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.

.github/workflows/test.yml

@@ -72,10 +72,19 @@ jobs:
       - name: Build editor-ext
         run: pnpm --filter @docmost/editor-ext build
 
+      # @docmost/prosemirror-markdown is the shared converter (#293/#326); its
+      # build/ is gitignored, and plain `pnpm -r test` does NOT honour nx
+      # `dependsOn: ^build`, so its consumers (mcp `pretest: tsc`, git-sync vitest
+      # typecheck) fail with TS2307 Cannot find module '@docmost/prosemirror-markdown'
+      # unless it is built first. Build it BEFORE git-sync/mcp (which import it).
+      - name: Build prosemirror-markdown
+        run: pnpm --filter @docmost/prosemirror-markdown build
+
       # git-sync and mcp are no longer committed in built form (build/ is
       # gitignored), so CI must compile them: the server resolves both via their
-      # built build/index.js. The server pretest also builds them, but building
-      # here keeps it explicit and independent of pnpm lifecycle ordering.
+      # built build/index.js (git-sync's runtime consumer lands on this branch,
+      # #119). The server pretest also builds them, but building here keeps it
+      # explicit and independent of pnpm lifecycle ordering.
       - name: Build git-sync and mcp
         run: pnpm --filter @docmost/git-sync build && pnpm --filter @docmost/mcp build