fix: expose pi mcp extension as mcps.ts (#315)

## Summary
- add project-local `mcps.ts` extension entry for Pi auto-discovery
- move MCP bridge implementation into `.pi/extensions/mcps/main.ts`
- ignore nested `node_modules/` directories so extension package
installs do not dirty repo

## Testing
- `node -e "const
createJiti=require('/opt/homebrew/lib/node_modules/@mariozechner/pi-coding-agent/node_modules/@mariozechner/jiti/lib/jiti.cjs');
const jiti=createJiti(process.cwd()); const
mod=jiti('./.pi/extensions/mcps.ts'); if (typeof mod.default !==
"function") process.exit(1); console.log("ok");"`

.gitignore

@@ -1,6 +1,7 @@
 /.phpunit.cache
 /bootstrap/ssr
 /node_modules
+node_modules/
 /public/build
 /public/hot
 /public/storage

.pi/extensions/mcps.ts

@@ -0,0 +1 @@
+export { default } from './mcps/main.ts';

.pi/extensions/mcps/package.json

@@ -0,0 +1,11 @@
+{
+    "name": "pi-mcp-bridge",
+    "private": true,
+    "description": "Project-local Pi extension that loads MCP servers from .mcp.json or opencode.json",
+    "type": "module",
+    "dependencies": {
+        "@modelcontextprotocol/sdk": "^1.29.0",
+        "@sinclair/typebox": "^0.34.49",
+        "zod": "^4.3.6"
+    }
+}