feat: add hot-swappable executor support for collections and agents

- Add executor configuration to Collection and Agent models in Prisma schema
- Create ExecutorConfigPanel component for selecting default or custom executors
- Add executor resolution logic with cascade (Agent → Collection → System Default)
- Create /api/executors/verify endpoint to test custom executor connectivity
- Add executor documentation page at /docs/executors with API specification
- Create deployable Vercel executor template in templates/vercel-executor/
- Update MCP handlers and agent tool execution to use configurable executors
- Add executor types and schemas to @tpmjs/types package

Users can now deploy their own executor instances and configure collections
or agents to use custom executors instead of the TPMJS default executor.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

templates/vercel-executor/vercel.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "framework": "nextjs",
+  "headers": [
+    {
+      "source": "/api/(.*)",
+      "headers": [
+        { "key": "Access-Control-Allow-Origin", "value": "*" },
+        { "key": "Access-Control-Allow-Methods", "value": "GET, POST, OPTIONS" },
+        { "key": "Access-Control-Allow-Headers", "value": "Content-Type, Authorization" }
+      ]
+    }
+  ]
+}