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>

packages/db/prisma/schema.prisma

@@ -422,6 +422,10 @@ model Collection {
   isPublic    Boolean  @default(false) @map("is_public")
   likeCount   Int      @default(0) @map("like_count")
 
+  // Executor configuration (optional - uses system default if not set)
+  executorType   String? @map("executor_type") @db.VarChar(50)
+  executorConfig Json?   @map("executor_config") @db.JsonB
+
   // Timestamps
   createdAt   DateTime @default(now()) @map("created_at")
   updatedAt   DateTime @updatedAt @map("updated_at")
@@ -513,6 +517,11 @@ model Agent {
   isPublic             Boolean    @default(true) @map("is_public")
   likeCount            Int        @default(0) @map("like_count")
 
+  // Executor configuration (optional - uses system default if not set)
+  // Agent executor overrides collection executor overrides system default
+  executorType         String?    @map("executor_type") @db.VarChar(50)
+  executorConfig       Json?      @map("executor_config") @db.JsonB
+
   // Timestamps
   createdAt            DateTime   @default(now()) @map("created_at")
   updatedAt            DateTime   @updatedAt @map("updated_at")