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/package.json

@@ -23,7 +23,7 @@
     "migration:reset": "tsx src/database/migrate.ts down-to NO_MIGRATIONS",
     "migration:codegen": "kysely-codegen --dialect=postgres --camel-case --env-file=../../.env --out-file=./src/database/types/db.d.ts",
     "lint": "eslint \"{src,apps,libs,test}/**/*.ts\" --fix",
-    "pretest": "pnpm --filter @docmost/editor-ext build && pnpm --filter @docmost/git-sync build && pnpm --filter @docmost/mcp build",
+    "pretest": "pnpm --filter @docmost/editor-ext build && pnpm --filter @docmost/prosemirror-markdown build && pnpm --filter @docmost/git-sync build && pnpm --filter @docmost/mcp build",
     "test": "jest",
     "test:int": "jest --config test/jest-integration.json",
     "test:watch": "jest --watch",
@@ -215,6 +215,7 @@
       "^src/(.*)$": "<rootDir>/$1",
       "^@docmost/git-sync$": "<rootDir>/../../../packages/git-sync/src/index.ts",
       "^@docmost/git-sync/(.*)$": "<rootDir>/../../../packages/git-sync/src/$1",
+      "^@docmost/prosemirror-markdown$": "<rootDir>/../../../packages/prosemirror-markdown/src/index.ts",
       "^(\\.{1,2}/.*)\\.js$": "$1"
     }
   }