feat(context-refresh): add repo-intel source for GitHub history snapshots

[stackbilt-admin]

Summary

Closes #138.

Adds repo-intel as a new source for charter context-refresh --sources repo-intel. Uses the gh CLI to fetch open/closed issues, recent PRs, and release history, then writes a structured snapshot to .charter/repo-intel/snapshot.json and contributes a summary to .ai/context.adf.

Snapshot shape:

{
  "available": true,
  "summary": {
    "openIssueCount": 12,
    "stalledIssues": 3,
    "recurringLabels": ["bug"],
    "mergeVelocity": 7,
    "releaseCadence": 14
  }
}

Graceful degradation: if gh is unavailable or the repo has no GitHub remote, emits a warning and continues — never a hard failure.

Test plan

• Happy path: snapshot written with correct summary fields
• ADF output includes repo-intel section and sourcesUsed
• gh unavailable → warning emitted, no error, snapshot not written
• 5/5 existing context-refresh tests pass (no regressions)
• 3/3 new repo-intel tests pass
• TypeScript clean

🤖 Generated with Claude Code

[stackbilt-admin]

Review pass complete for #182.

No blocking issues found in the current diff.

What I validated:

• repo-intel source degrades gracefully when gh is unavailable.
• Source inclusion in sourcesUsed and derived aggregates is wired coherently.
• Full repo-intel snapshot persistence and tests are present and aligned with behavior.

Non-blocking follow-up suggestion:

• Consider whether .charter/repo-intel/snapshot.json should respect custom --config paths, or remain intentionally fixed at .charter and be documented as such.

I’m comfortable merging this as-is.