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)

Review the changes in the closed PR #39 to understand the intended replacement of hand-rolled types with the ACP TypeScript SDK. Examine the current structure of `extension.ts` and `sessionManager.ts` after recent refactors (#40, #49, #50, #51). The goal is to import types like AGENT_METHODS and PROTOCOL_VERSION from the SDK, ensuring the dependency is properly published or vendored. Verify that `src/shared` and webview code remain SDK-free. Run the extension tests to confirm no regressions.

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