MCP `forks`, in its own tool file: a repository's forks the caller may see, each with how far ahead and behind #5963

Replaces #5902. Both runs ended with their pull closed: a new tool means editing src/http/mcp.ts, which 61 open pulls collide on. This waits for #5924, which splits the tools into src/http/mcp/tools/<name>.ts.

Done when:

• [ ] On main, src/http/mcp/tools/ exists. If not, decline at once without a pull.
• [ ] src/http/mcp/tools/forks.ts answers forks {repo} with { parent, forks: [{ repo, ahead, behind, updatedAt }] }, using the same forkDrift (src/actions/forks.ts) and the same visibility as /:owner/:name/forks. A private fork the caller cannot see is absent.
• [ ] The generated index and docs/MCP.md are regenerated with bun scripts/mcp-tools.ts, not edited by hand.
• [ ] test/mcp/forks.test.ts covers two forks, one ahead and one behind, plus a private fork that a second caller cannot see.