agents/bundles: reactors must read metadata or be defaults

The left4me bundle's first cut had two reactors that returned
static dicts without calling metadata.get(...): systemd_services
(enable/run flags) and nftables_output (two static rule strings).
Both passed bw test (no consumer yet). Once attached to
ovh.left4me, bw raised "did not request any metadata, you might
want to use defaults instead". Fix was to fold both into defaults.

Document the pitfall, with the verbatim error wording and the
note that this applies to cross-namespace contributions too.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

bundles/AGENTS.md

@@ -110,6 +110,14 @@ bundles/<name>/
   itself; grep `'<other-bundle>':` in the reactors when in doubt.
 - **`bw hash` doesn't accept selectors.** Use `bw hash <node>` per
   literal name; see the fork's runbook.
+- **Reactors must read metadata.** If a reactor body returns a static
+  dict without calling `metadata.get(...)`, bw raises
+  `ValueError: <reactor> on <node> did not request any metadata, you
+  might want to use defaults instead` once a node consumes the bundle.
+  Fix: fold the contribution into `defaults`. The rule applies even
+  when the reactor writes into another bundle's namespace — a static
+  contribution to e.g. `nftables/output` belongs in `defaults`, where
+  bw merges it with other bundles' contributions.
 
 ## Per-bundle README