docs: document the independent subagent daemon (start/continue/interrupt)

docs/tools/README.md

@@ -28,6 +28,10 @@ debug agent behavior.
 
 - **[bash](bash.md)** — background shell execution (`mcp__bash__*`),
   available on every agent unconditionally.
+- **[subagent](subagent.md)** — spawn nested headless claude sessions
+  (`mcp__subagent__{start,continue,interrupt}`), shipped default-on for
+  every agent today alongside `bash` (expected to become a real opt-in
+  capability later).
 - **[forge](forge.md)** — the `hive-forge` Forgejo CLI every agent has
   for issues, PRs, and comments. Not an MCP tool — a binary agents
   shell out to instead of ad-hoc curl.