fix: feat: gardener bundles dust into ore before promoting to backlog (#74)

- Add dust/ore rule to gardener LLM prompt: trivial tech-debt (comment
  fix, rename, style-only, single-line) outputs DUST: JSON instead of
  promoting individually
- Parse DUST lines from LLM output, validate JSON, append to dust.jsonl
  with timestamp
- After evaluation pass: check groups with 3+ items, create bundled
  backlog issue, close source issues with cross-reference
- Add gardener/dust.jsonl to .gitignore
- Create gardener/PROMPT.md documenting the dust vs ore philosophy

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

gardener/PROMPT.md

@@ -0,0 +1,46 @@
+# Gardener Prompt — Dust vs Ore
+
+## Rule
+
+Don't promote trivial tech-debt individually. Each promotion costs a full
+factory cycle: CI + dev-agent + review + merge. Don't fill minecarts with
+dust — put ore inside.
+
+## What is dust?
+
+- Comment fix
+- Variable rename
+- Style-only change (whitespace, formatting)
+- Single-line edit
+- Trivial cleanup with no behavioral impact
+
+## What is ore?
+
+- Multi-file changes
+- Behavioral fixes
+- Architectural improvements
+- Security or correctness issues
+- Anything requiring design thought
+
+## LLM output format
+
+When a tech-debt issue is dust, the LLM outputs:
+
+```
+DUST: {"issue": NNN, "group": "<file-or-subsystem>", "title": "...", "reason": "..."}
+```
+
+The `group` field clusters related dust by file or subsystem (e.g.
+`"gardener"`, `"lib/env.sh"`, `"dev-poll"`).
+
+## Bundling
+
+The script collects dust items into `gardener/dust.jsonl`. When a group
+accumulates 3+ items, the script automatically:
+
+1. Creates one bundled backlog issue referencing all source issues
+2. Closes the individual source issues with a cross-reference comment
+3. Removes bundled items from the staging file
+
+This converts N trivial issues into 1 actionable issue, saving N-1 factory
+cycles.