Add HIIT watch runner, rest-time setting, and HealthKit watch auto-launch

- Redesign the watch app into an active-workout runner: a root gate shows the
  in-progress workout's exercises or prompts to start one on iPhone, and each
  exercise runs as a horizontally-paged HIIT cycle (count-up work, count-down
  rest with final-three-second haptics + auto-advance, One More / Done on the
  last set). Replaces the old history list.
- Add a configurable rest-between-sets duration in iPhone Settings (default 45s),
  synced to the watch over WatchConnectivity.
- Launch the watch app into the session when a workout starts on the phone via
  HealthKit (startWatchApp); the watch runs an HKWorkoutSession for foreground
  runtime and ends it when the workout finishes. Adds the HealthKit entitlement +
  Health usage strings on both targets and WKBackgroundModes on the watch.

Claude-Session: https://claude.ai/code/session_018gg69MaUetDNzWzBXisfMV

REQUIREMENTS.md

@@ -47,6 +47,10 @@ local SwiftData cache fed only by WatchConnectivity:
   every cache change.
 - Watch → Phone: sends an updated `WorkoutDocument` (keyed by ULID); the phone
   applies it through the file write path (the sole writer of iCloud Drive).
+- Starting a workout on the phone launches the watch app into it via HealthKit
+  (`startWatchApp(toHandle:)`); the watch runs an `HKWorkoutSession` for foreground
+  runtime and ends it when no active workout remains. Requires the HealthKit
+  capability and a one-time Health permission on both devices.
 
 ## Seed Data
 Starter splits (Upper Body / Core / Lower Body) are generated on demand from the
@@ -60,7 +64,8 @@ auto-seeded. The catalogs also back the in-workout exercise picker.
 - Start a workout from a split; track per-exercise set/rest/done progress; mark
   complete/skipped; edit the plan during a session (mirrored back to the split).
 - Weight-progression charts per exercise across past sessions.
-- Apple Watch: run workouts from the wrist; changes sync back to the phone.
+- Apple Watch: starting a workout on the phone launches the watch into it; run the
+  session from the wrist (HIIT-style work/rest cycles); changes sync back to the phone.
 
 ## Dependencies
 - **Yams** — YAML parsing for the exercise catalogs.