chore(git-sync): stop committing build/ and node_modules; build in CI/Docker

Review finding #2: packages/git-sync/build/ (the COMPILED engine) and the
package's node_modules/ were committed. Prod executed the committed build/ while
CI/tests ran src/ and never rebuilt it — so a fix in src/ could pass tests while
stale compiled code shipped (a silent src/prod skew). The committed node_modules
were pnpm symlinks with a baked machine-local store path (/home/claude/...),
useless and misleading for everyone else.

- git rm --cached packages/git-sync/{build,node_modules} (42 + 31 files).
- .gitignore: ignore packages/*/node_modules/ and packages/git-sync/build/.
- Build the package where it is actually consumed: apps/server `pretest` now
  builds @docmost/git-sync (its suite imports the built build/index.js), and the
  CI Test workflow gains an explicit "Build git-sync" step. The Dockerfile builder
  already runs `pnpm build` (nx builds the package) and now COPYs the fresh build/.

Verified: wiped build/, rebuilt via `pnpm --filter @docmost/git-sync build`, then
the server converter gate (26/26, imports the rebuilt package) and the git-sync
suite (588 passed) both pass against the freshly-built, non-committed output.

NOTE: packages/mcp/ has the same committed-build/node_modules pattern (pre-existing,
out of this PR's scope) and should get the same treatment in a follow-up.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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",
+    "pretest": "pnpm --filter @docmost/editor-ext build && pnpm --filter @docmost/git-sync build",
     "test": "jest",
     "test:int": "jest --config test/jest-integration.json",
     "test:watch": "jest --watch",