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>

README.md

@@ -128,11 +128,9 @@ disinto/
 │   ├── vault-poll.sh     # Cron entry: process pending dangerous actions
 │   ├── vault-agent.sh    # Classifies and routes actions (claude -p)
 │   ├── vault-fire.sh     # Executes an approved action
-│   ├── vault-reject.sh   # Marks an action as rejected
-│   └── PROMPT.md         # System prompt for vault agent
+│   └── vault-reject.sh   # Marks an action as rejected
 └── supervisor/
     ├── supervisor-poll.sh   # Supervisor: health checks + claude -p
-    ├── PROMPT.md         # Supervisor's system prompt
     ├── update-prompt.sh  # Self-learning: append to best-practices
     └── best-practices/   # Progressive disclosure knowledge base
         ├── memory.md