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>

vault/vault-agent.sh

@@ -15,7 +15,7 @@ source "${SCRIPT_DIR}/vault-env.sh"
 
 VAULT_SCRIPT_DIR="${FACTORY_ROOT}/vault"
 OPS_VAULT_DIR="${OPS_REPO_ROOT}/vault"
-PROMPT_FILE="${VAULT_SCRIPT_DIR}/PROMPT.md"
+PROMPT_FILE="${FACTORY_ROOT}/formulas/run-vault.toml"
 LOGFILE="${VAULT_SCRIPT_DIR}/vault.log"
 CLAUDE_TIMEOUT="${CLAUDE_TIMEOUT:-3600}"