* feat(context): include diffHunk in PR review comment context
Review comments arrived with only path and line, so the code they were
written against was missing from the prompt. Fetch diffHunk in the PR
GraphQL query and render it under the comment as a diff block.
The hunk is PR-authored content, so it goes through sanitizeContent like
the comment body. Comments without a hunk are unchanged.
Fixes#855
* test(formatter): cover outdated review comments with an empty diff hunk
GitHub returns diffHunk: "" (not null) for comments whose line no longer
exists in the diff, so the render guard has to reject empty strings too.
Found running the real query against anthropics/claude-code-action#1025.
The non-signing path validated newBranch before checkout, but the
use_commit_signing path passed it straight to the file ops server, so an
invalid branch_name_template surfaced only as a 422 "Reference name is
not valid" on the first commit.
Validate once after the name is resolved so both paths fail early with
the same message.
Fixes#1573
