feat(rpc): handle extension requests directly

[code-yeongyu]

Problem

Extension RPC was one-way: extensions could emit capability-gated state, but an RPC client could
not invoke extension-owned controls without turning them into slash-command prompts.

Fix

• add generation-owned pi.rpc.handle(name, handler) registration
• add the session-scoped extension_request JSONL command and structured response
• expose RpcClient.requestExtension()
• reject empty, unknown, duplicate, and stale-generation handlers
• preserve existing multi-session routing and response tagging
• document the public extension and wire contracts

Verification

• npm --prefix packages/coding-agent test -- test/suite/rpc-extension-requests.test.ts test/suite/rpc-extension-events.test.ts test/rpc-multi-session.test.ts
• npx tsc --noEmit --pretty false
• npm run build
• npm run check
• mutation proof: forcing a beta request into the alpha binding fails the new isolation assertion

Model: gpt-5.6-sol-fast
Harness: omo-senpi

---

Summary by cubic

Enables direct client-to-extension RPC requests so clients can call extension-owned handlers without the model. Adds a session-scoped RPC command and client API with strict validation, multi-session tagging, and rejection of stale or in-flight results after reloads.

• New Features

  • pi.rpc.handle(name, handler) to register one handler per name on the active extension generation; rejects empty, unknown, duplicate, and stale-generation cases.
  • New RPC command extension_request with structured success/error responses; data is opaque; responses include sessionId in multi-session mode.
  • RpcClient.requestExtension(name, data?) to call a handler and get its result.
  • Docs updated; tests cover handler routing, multi-session isolation, and generation liveness.

• Bug Fixes

  • Reject stale in-flight handler results by re-checking generation liveness after handler completion; invalidate the previous ExtensionRunner after reload notifications.
  • pi.rpc.emit() now asserts generation liveness before publishing.

^{Written for commit 901171b. Summary will update on new commits.}