idle: EWMA activity tracking

Track activity level as an EWMA (exponentially weighted moving average)
driven by turn duration. Long turns (engaged work) produce large boosts;
short turns (bored responses) barely register.

Asymmetric time constants: 60s boost half-life for fast wake-up, 5-minute
decay half-life for gradual wind-down. Self-limiting boost formula
converges toward 0.75 target — can't overshoot.

- Add activity_ewma, turn_start, last_nudge to persisted state
- Boost on handle_response proportional to turn duration
- Decay on every tick and state transition
- Fix kent_present: self-nudge responses (fired=true) don't update
  last_user_msg, so kent_present stays false during autonomous mode
- Nudge only when Kent is away, minimum 15s between nudges
- CLI: `poc-daemon ewma [VALUE]` to query or set
- Status output shows activity percentage

src/bin/poc-daemon/rpc.rs

@@ -166,6 +166,17 @@ impl daemon::Server for DaemonImpl {
         Promise::ok(())
     }
 
+    fn ewma(
+        &mut self,
+        params: daemon::EwmaParams,
+        mut results: daemon::EwmaResults,
+    ) -> Promise<(), capnp::Error> {
+        let value = pry!(params.get()).get_value();
+        let current = self.state.borrow_mut().handle_ewma(value);
+        results.get().set_current(current);
+        Promise::ok(())
+    }
+
     fn stop(
         &mut self,
         _params: daemon::StopParams,
@@ -211,6 +222,7 @@ impl daemon::Server for DaemonImpl {
         status.set_since_activity(s.since_activity());
         status.set_since_user(crate::now() - s.last_user_msg);
         status.set_block_reason(s.block_reason());
+        status.set_activity_ewma(s.activity_ewma);
 
         Promise::ok(())
     }