Ava: add GitHub read tools (get_pr, get_issue, get_commit_diff)

[mabry1985]

Gap

Ava can write GitHub issues (create_github_issue) but cannot read individual PRs, issues, or commits by number from arbitrary managed repos. The only read surface is get_pr_pipeline, which is a snapshot of currently open PRs.

Triggering case

Operator asked: "Confirm commit 6d798306359b77134a273618c6aac2dfafccd587 (from PR #3671) resolved issues #3689, #3684, #3702."

Ava could not:

• Fetch the PR description / linked issues
• Read the commit diff
• Check whether the referenced issues are open or closed
• Compare the fix surface against the bug reports

Proposed tools

• github_get_pr(repo, number) → title, body, state, linked issues, head sha, merge commit
• github_get_issue(repo, number) → title, body, state, labels, comments
• github_get_commit(repo, sha) → message, author, files changed, diff
• github_search_issues(repo, query) → for fuzzy lookup when repo is unknown

Why this matters

Verification work ("did the fix actually cover the bug?") is bread-and-butter chief-of-staff work. Without read access, Ava either fabricates or punts back to the operator. Both are bad.

Acceptance

Operator can ask Ava to verify any fix against any issue in a managed repo and get a grounded answer with diff-level evidence.