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

schema/daemon.capnp

@@ -44,6 +44,7 @@ struct Status {
   sinceActivity  @14 :Float64;   # secs since max(lastUserMsg, lastResponse)
   sinceUser      @15 :Float64;   # secs since lastUserMsg
   blockReason    @16 :Text;      # why idle timer hasn't fired
+  activityEwma   @17 :Float64;   # 0-1, EWMA of recent activity (running fraction)
 }
 
 interface Daemon {
@@ -71,6 +72,8 @@ interface Daemon {
   save             @17 () -> ();
   debug            @18 () -> (json :Text);
 
+  ewma             @20 (value :Float64) -> (current :Float64);
+
   # Modules
   moduleCommand    @15 (module :Text, command :Text, args :List(Text))
                        -> (result :Text);