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>

.github/workflows/test.yml

@@ -36,12 +36,12 @@ jobs:
       - name: Build editor-ext
         run: pnpm --filter @docmost/editor-ext build
 
-      # git-sync is no longer committed in built form (build/ is gitignored), so
-      # CI must compile it: the server suite imports the package via its built
-      # build/index.js. The server pretest also builds it, but building here keeps
-      # it explicit and independent of pnpm lifecycle ordering.
-      - name: Build git-sync
-        run: pnpm --filter @docmost/git-sync 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.
+      - name: Build git-sync and mcp
+        run: pnpm --filter @docmost/git-sync build && pnpm --filter @docmost/mcp build
 
       - name: Run tests
         run: pnpm -r test