fix(mcp): bundle build with esbuild so plugin cache never needs node_modules

Claude Code spawns plugin MCP servers straight from the read-only
git-content plugin cache, which never gets an npm install for mcp/. The
old tsc-multi-file build imported bare specifiers that only resolved
via mcp/node_modules, so every fresh plugin install/update produced a
-32000 reconnect failure. Bundle mcp/src into a single dependency-free
build/index.js instead; the one CJS-heavy import (express, only used
by --transport=http) is now dynamic so the stdio path never evaluates it.

docs/MCP.md

@@ -152,6 +152,15 @@ no "not ready yet, retry" state, so an async bootstrap cannot win that race. The
 build artifact is therefore committed, and `plugins/ccam/.mcp.json` points at
 `${CLAUDE_PLUGIN_ROOT}/mcp/build/index.js`.
 
+`npm run mcp:build` bundles `mcp/src` and its runtime dependencies
+(`esbuild --bundle`) into that single `build/index.js` file — the plugin cache
+never gets an `npm install` for `mcp/`, so a build that still had bare
+`node_modules` imports (plain `tsc` output) would throw `MODULE_NOT_FOUND` the
+instant Claude Code spawned it after any plugin install or update. `express`
+(only used by `--transport=http`, never by the stdio mode a real MCP host
+uses) is pulled in lazily via `await import(...)` in `src/index.ts` instead of
+a static import, so the stdio path never even evaluates it.
+
 The cost is drift, so freshness is enforced by content hash — `mcp/src` plus the
 MCP manifests and tsconfig are hashed into `mcp/build/.srchash`:
 
@@ -179,8 +188,9 @@ mcp/
 │   │   └── stats.ts          # Statistics tools
 │   └── types.ts              # TypeScript type definitions
 │
-├── build/                    # Compiled JavaScript (COMMITTED — see above)
-│   └── .srchash              # hash of mcp/src the build was produced from
+├── build/                    # Bundled, dependency-free output (COMMITTED — see above)
+│   ├── index.js               # single esbuild bundle, no node_modules needed to run it
+│   └── .srchash               # hash of mcp/src the build was produced from
 ├── package.json
 ├── tsconfig.json
 └── README.md