feat: add issue lifecycle workflows

[chaodu-agent]

Summary

Adds three workflows to manage the issue label lifecycle, mirroring the PR lifecycle (#450, #451, #452).

New Workflows

1. issue-pending-maintainer.yml

Flips pending-contributor → pending-maintainer when the issue author replies.

Triggers: issue_comment (instant) + hourly schedule (safety net) + workflow_dispatch

Guard conditions:

• No closing-soon
• No wontfix / not-planned
• Last human comment is from issue author (bot comments filtered)

2. stale-issue.yml

Escalates stale issues to closing-soon.

Trigger: Hourly + workflow_dispatch

Behavior: pending-contributor applied for ≥ 7 days → adds closing-soon

3. close-stale-issues.yml

Auto-closes issues that have been closing-soon for too long.

Trigger: Daily at 09:00 UTC (same as close-stale-prs.yml) + workflow_dispatch

Behavior: closing-soon for ≥ 3 days → auto-close with state_reason: not_planned

Issue Lifecycle

Issue opened
  → +needs-triage (existing issue-triage.yml)
  → +incomplete if template missing (existing issue-check.yml)
  │
  ├─ pending-contributor (needs info from author)
  │   ├─ author replies → flip to pending-maintainer (this PR)
  │   └─ 7 days no reply → +closing-soon → 3 days → auto-close
  │
  └─ info complete → maintainer processes

Comparison with PR Lifecycle

	PRs	Issues
Stale threshold	2 days	7 days
closing-soon → close	3 days	3 days
Total worst case	5 days	10 days

Files Changed

• .github/workflows/issue-pending-maintainer.yml (new)
• .github/workflows/stale-issue.yml (new)
• .github/workflows/close-stale-issues.yml (new)

[github-actions]

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