refactor: replace MCP tools with session CLI transport

[benvinegar]

Summary

• remove Hunk's agent-facing MCP tool surface and replace it with a small session-oriented daemon API
• keep hunk session ... as the stable user/agent interface for live review control
• update docs, Pi skill guidance, and end-to-end coverage to be CLI-first

What changed

• added src/session/protocol.ts for the daemon's JSON session API and capabilities endpoint
• rewrote src/session/commands.ts to talk directly to that session API instead of the MCP SDK client
• updated src/mcp/server.ts to:
  • serve /session-api
  • serve /session-api/capabilities
  • return a clear 410 error on the old /mcp route telling callers to use hunk session ...
• removed @modelcontextprotocol/sdk from package.json
• updated README and bundled Pi skill docs to steer agents toward hunk session ...
• rewrote the live end-to-end coverage to drive real Hunk sessions through the session CLI
• added server coverage for capabilities and for rejecting the old MCP endpoint

User impact

From a normal user's CLI perspective, the hunk session ... commands stay the same.

The change is underneath that surface:

• before: the session CLI was a thin client over Hunk's MCP tools
• now: the session CLI is a thin client over Hunk's own session API

Validation

• bun run typecheck
• bun test
• bun run check:pack