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>

schema/daemon.capnp

@@ -61,6 +61,10 @@ interface Daemon {
   getTypes         @13 () -> (types :List(TypeInfo));
   setThreshold     @14 (type :Text, level :UInt8) -> ();
 
+  idleTimeout      @16 (seconds :Float64) -> ();
+  save             @17 () -> ();
+  debug            @18 () -> (json :Text);
+
   # Modules
   moduleCommand    @15 (module :Text, command :Text, args :List(Text))
                        -> (result :Text);