feat: supervisor detects dep deadlocks, stale deps, and dev-agent blocked states

Add three new supervisor checks:
- P2c: alert when dev-agent reports "no ready issues" for 6+ consecutive polls
- P3b: detect circular dependency deadlocks via DFS cycle detection
- P3c: flag backlog issues blocked by deps open >30 days

Update supervisor PROMPT.md with guidance for Claude to resolve circular deps
by reading code context, and handle stale deps by checking relevance.

Gardener prompt now forbids bidirectional deps between sibling issues and
requires ## Related (not ## Dependencies) for cross-references.

Closes #16, Closes #17

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

gardener/gardener-poll.sh

@@ -196,6 +196,7 @@ Process up to 10 tech-debt issues per run (stay within API rate limits).
 3. **Stale issues**: If clearly superseded or no longer relevant, close with explanation. If unclear, ESCALATE.
 4. **Oversized issues**: If >5 acceptance criteria touching different files/concerns, ESCALATE with suggested split.
 5. **Dependencies**: If an issue references another that must land first, add a \`## Dependencies\n- #NNN\` section if missing.
+6. **Sibling issues**: When creating multiple issues from the same source (PR review, code audit), NEVER add bidirectional dependencies between them. Siblings are independent work items, not parent/child. Use \`## Related\n- #NNN (sibling)\` for cross-references between siblings — NOT \`## Dependencies\`. The dev-poll \`get_deps()\` parser only reads \`## Dependencies\` / \`## Depends on\` / \`## Blocked by\` headers, so \`## Related\` is safely ignored. Bidirectional deps create permanent deadlocks that stall the entire factory.
 
 ## Escalation format
 For anything needing human decision, output EXACTLY this format (one block, all items):