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/schema-attribute-contract.spec.ts

@@ -1,10 +1,11 @@
 import { getSchema } from '@tiptap/core';
 import { Schema } from '@tiptap/pm/model';
 import { tiptapExtensions } from './collaboration.util';
-// The vendored git-sync mirror's extension set. Imported via the subpath the
-// server jest config maps to the package SOURCE (moduleNameMapper
-// `^@docmost/git-sync/(.*)$`), so this reads the real mirror, not a build.
-import { docmostExtensions as gitSyncExtensions } from '@docmost/git-sync/lib/docmost-schema';
+// The canonical converter mirror's extension set. The schema mirror now lives in
+// the single `@docmost/prosemirror-markdown` package (#293); the server jest
+// config maps it to the package SOURCE (moduleNameMapper
+// `^@docmost/prosemirror-markdown$`), so this reads the real mirror, not a build.
+import { docmostExtensions as gitSyncExtensions } from '@docmost/prosemirror-markdown';
 
 /**
  * ATTRIBUTE-LEVEL SCHEMA CONTRACT (review #293, variant A).