fix: address review findings from issue #74

- Add dedup guard: skip dust entries for issues already in dust.jsonl
- Inject already-staged issue list into LLM prompt to prevent re-emission
- Guard mv after jq: only overwrite dust.jsonl if jq succeeded
- Use sort -nu for numeric dedup of issue numbers
- Compute bundle count from distinct issues, not raw entries
- Add 30-day TTL expiry for sub-threshold dust groups
- Fix inconsistent heading levels in bundle body (all ###)
- Add scope note to PROMPT.md (human docs only, not injected)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gardener/PROMPT.md

@@ -1,5 +1,9 @@
 # Gardener Prompt — Dust vs Ore
 
+> **Note:** This is human documentation. The actual LLM prompt is built
+> inline in `gardener-poll.sh` (with dynamic context injection). This file
+> documents the design rationale for reference.
+
 ## Rule
 
 Don't promote trivial tech-debt individually. Each promotion costs a full