fix: reconcile resolve-pr-thread vs resolve-pr-review-thread naming

[jamesadevine]

Problem

The PR thread resolution tool has inconsistent naming across the codebase:

• AGENTS.md documents the tool as resolve-pr-thread
• MCP registration (src/mcp.rs:1036) uses resolve-pr-review-thread
• Executor dispatch (src/execute.rs:381) uses resolve-pr-review-thread
• Validation (common.rs validate_resolve_pr_thread_statuses()) uses resolve-pr-review-thread as the front matter key

Desired outcome

Standardise on resolve-pr-thread (the shorter, cleaner name matching the AGENTS.md documentation):

1. Rename the MCP tool registration from resolve-pr-review-thread → resolve-pr-thread
2. Update the executor dispatch key to match
3. Update the validation function's front matter key to match
4. Verify AGENTS.md already uses the correct name (it does)

Notes

This is a breaking change for any existing front matter using resolve-pr-review-thread: in safe-outputs:. Consider accepting both names during a deprecation period, or just change it since this tool is new.