idle: persist timeout, suppress restart fire, add debug/save/idle-timeout commands

Several idle timer fixes and new inspection capabilities:

- Persist idle_timeout across daemon restarts (was reverting to 5min default)
- Set fired=true on load to suppress immediate fire from stale timestamps
- Add human-readable ISO timestamps to daemon-state.json for debugging
- Use to_string_pretty for readable state file
- Make save() public for RPC access
- Remove kb_idle_minutes() — go purely off message timestamps
- Add maybe_prompt_notification() with idle gate so notifications only
  inject via tmux when truly idle, not during active sessions
- Add debug_json() for full state inspection with computed values
  (would_fire, block_reason, all timers)

New RPC commands (schema @16-18):
  poc-daemon idle-timeout <secs>  — set idle timeout
  poc-daemon save                 — force state persistence
  poc-daemon debug                — dump full internal state as JSON

Also: save state on clean shutdown, route module notifications through
maybe_prompt_notification before submitting to queue.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

src/bin/poc-daemon/rpc.rs

@@ -126,6 +126,36 @@ impl daemon::Server for DaemonImpl {
         Promise::ok(())
     }
 
+    fn idle_timeout(
+        &mut self,
+        params: daemon::IdleTimeoutParams,
+        _results: daemon::IdleTimeoutResults,
+    ) -> Promise<(), capnp::Error> {
+        let secs = pry!(params.get()).get_seconds();
+        self.state.borrow_mut().handle_idle_timeout(secs);
+        Promise::ok(())
+    }
+
+    fn save(
+        &mut self,
+        _params: daemon::SaveParams,
+        _results: daemon::SaveResults,
+    ) -> Promise<(), capnp::Error> {
+        self.state.borrow().save();
+        info!("state saved");
+        Promise::ok(())
+    }
+
+    fn debug(
+        &mut self,
+        _params: daemon::DebugParams,
+        mut results: daemon::DebugResults,
+    ) -> Promise<(), capnp::Error> {
+        let json = self.state.borrow().debug_json();
+        results.get().set_json(&json);
+        Promise::ok(())
+    }
+
     fn stop(
         &mut self,
         _params: daemon::StopParams,