Add hooks system with pi.send() for external message injection

- Hook discovery from ~/.pi/agent/hooks/, .pi/hooks/, --hook flag
- Events: session_start, session_switch, agent_start/end, turn_start/end, tool_call, tool_result, branch
- tool_call can block execution, tool_result can modify results
- pi.send(text, attachments?) to inject messages from external sources
- UI primitives: ctx.ui.select/confirm/input/notify
- Context: ctx.exec(), ctx.cwd, ctx.sessionFile, ctx.hasUI
- Docs shipped with npm package and binary builds
- System prompt references docs folder

packages/ai/src/agent/index.ts

@@ -1,3 +1,3 @@
 export { agentLoop, agentLoopContinue } from "./agent-loop.js";
 export * from "./tools/index.js";
-export type { AgentContext, AgentEvent, AgentLoopConfig, AgentTool, QueuedMessage } from "./types.js";
+export type { AgentContext, AgentEvent, AgentLoopConfig, AgentTool, AgentToolResult, QueuedMessage } from "./types.js";