Add respond-to-pr-comments CLI skill (#245)

Self-contained skill for addressing PR review feedback interactively.
Reads all review threads (with pagination), validates each comment,
proposes fixes or explanations, applies changes with user confirmation,
and resolves threads via GitHub API.

Key features:
- GraphQL-based thread fetching with cursor pagination
- Contradiction detection across reviewers
- Per-thread validity assessment (valid vs bikeshedding)
- Batched code fixes with single commit+push
- Mandatory user confirmation before every mutation

Co-authored-by: Alan Jowett <alan.jowett@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>