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>

supervisor/supervisor-poll.sh

@@ -19,7 +19,7 @@ source "$(dirname "$0")/../lib/ci-helpers.sh"
 LOGFILE="${DISINTO_LOG_DIR}/supervisor/supervisor.log"
 STATUSFILE="/tmp/supervisor-status"
 LOCKFILE="/tmp/supervisor-poll.lock"
-PROMPT_FILE="${FACTORY_ROOT}/supervisor/PROMPT.md"
+PROMPT_FILE="${FACTORY_ROOT}/formulas/run-supervisor.toml"
 PROJECTS_DIR="${FACTORY_ROOT}/projects"
 
 METRICS_FILE="${DISINTO_LOG_DIR}/metrics/supervisor-metrics.jsonl"