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/main.rs

@@ -104,6 +104,11 @@ enum Command {
     DreamEnd,
     /// Force state persistence to disk
     Save,
+    /// Get or set the activity EWMA (0.0-1.0). No value = query.
+    Ewma {
+        /// Value to set (omit to query)
+        value: Option<f64>,
+    },
     /// Dump full internal state as JSON
     Debug,
     /// Shut down daemon
@@ -202,9 +207,10 @@ async fn client_main(cmd: Command) -> Result<(), Box<dyn std::error::Error>> {
                         fmt_secs(s.get_since_activity()),
                         fmt_secs(s.get_notify_timeout()),
                     );
-                    println!("kent: {} (last {})",
+                    println!("kent: {} (last {})  activity: {:.1}%",
                         if s.get_kent_present() { "present" } else { "away" },
                         fmt_secs(s.get_since_user()),
+                        s.get_activity_ewma() * 100.0,
                     );
 
                     let sleep = s.get_sleep_until();
@@ -271,6 +277,13 @@ async fn client_main(cmd: Command) -> Result<(), Box<dyn std::error::Error>> {
                     daemon.save_request().send().promise.await?;
                     println!("state saved");
                 }
+                Command::Ewma { value } => {
+                    let mut req = daemon.ewma_request();
+                    req.get().set_value(value.unwrap_or(-1.0));
+                    let reply = req.send().promise.await?;
+                    let current = reply.get()?.get_current();
+                    println!("{:.1}%", current * 100.0);
+                }
                 Command::Debug => {
                     let reply = daemon.debug_request().send().promise.await?;
                     let json = reply.get()?.get_json()?.to_str()?;