chore(extension): adopt ACP TypeScript SDK for wire types

Extracted from #39, which replaces the hand-rolled ACP wire types with `@agentclientprotocol/sdk` imports (AGENT_METHODS, PROTOCOL_VERSION, InitializeResponse, SessionNotification, SessionModeState, ListSessionsResponse, etc.) in `extension.ts` and `sessionManager.ts`. As submitted it pins the dependency as `file:../acp-typescript-sdk`, which cannot install in CI and accounts for ~8k of the PR's lockfile churn.

Decision first: published npm pin vs vendored types vs staying hand-rolled. If adopted, preserve the boundary #39 already keeps: `src/shared` and the webview stay SDK-free. No user-visible change. Unblocks the session mode/model and session/list issues.

Note: `extension.ts` was restructured by #40/#49/#50/#51 since #39 was cut, so treat the PR hunks as a behavioral spec, not a patch.

**Size**: M | **Depends on**: nothing | **Tier**: 1 (foundation)