chore(mcp): stop committing build/ and node_modules; build in CI/Docker

Same hygiene fix as git-sync (review #2), applied to packages/mcp which had the
identical pre-existing problem: committed build/ (20 files) + node_modules (28,
pnpm symlinks with a baked /home/claude store path).

- git rm --cached packages/mcp/{build,node_modules}.
- .gitignore: add packages/mcp/build/ (packages/*/node_modules/ already covers it).
- Build where consumed: apps/server `pretest` and the CI Test workflow now build
  @docmost/mcp too. The Dockerfile builder already runs `pnpm build` (nx builds
  mcp) and already COPYs packages/mcp/build into the runtime image.

Verified: wiped build/, rebuilt via `pnpm --filter @docmost/mcp build`; the mcp
server suites (96 tests) pass against the freshly-built, non-committed output.

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 && pnpm --filter @docmost/git-sync build",
+    "pretest": "pnpm --filter @docmost/editor-ext 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",