fix: remove PROMPT.md files — formulas are the source of truth (#12)

- Delete gardener/PROMPT.md (dust-vs-ore rules already in run-gardener.toml)
- Delete supervisor/PROMPT.md (content covered by run-supervisor.toml;
  migrate unique "Learning" section into formula's journal step)
- Delete vault/PROMPT.md and create formulas/run-vault.toml as the
  source-of-truth formula for vault action classification/routing
- Update supervisor/supervisor-poll.sh to read from formula instead of PROMPT.md
- Update vault/vault-agent.sh to read from formula instead of PROMPT.md
- Update supervisor/AGENTS.md, vault/AGENTS.md, README.md references

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

gardener/PROMPT.md

@@ -1,50 +0,0 @@
-# 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
-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.