MCP `pulls`: list a repository's open pulls, briefly, with each one's check and review state #3401

MCP has pull (one pull) but no way to list them, so an agent in another harness has to page the REST API or guess numbers.

Done:

• [ ] A pulls tool takes repo (owner/name), optional state (open|closed, default open) and limit (1-50, default 20), validated at the boundary.
• [ ] Each item is one line of data: number, title, author, head branch, check verdict, approvals and changes requested, mergeable; no bodies, no diffs.
• [ ] A private repository the caller cannot read answers the same not-found as pull does.
• [ ] A test in the MCP tests lists pulls on a fixture, checks the brief shape and the not-found, and keeps tools/list under the 40 KB limit #3366 sets.

Files: src/http/mcp.ts (or the module that defines tools), its test, the Claude Code and OpenCode plugin tool lists if they enumerate tools. Waits for #3366 so the size budget is in place first.