feat: ephemeral timers with auto-cleanup and optional naming

[Faerkeren]

Summary

• Timers are ephemeral by default — HA helper is auto-created on first action and auto-deleted when the timer returns to idle, preventing accumulation of unused helpers
• client.timer() without a name auto-generates a unique ID (timer.haclient_a1b2c3d4)
• client.timer("name", persistent=True) keeps the HA helper alive after idle (requires explicit name)
• Same Timer object can be restarted after auto-cleanup — helper is transparently re-created

API Examples

# Ephemeral (default) — auto-created, auto-deleted on idle
t = client.timer()
await t.start(duration="00:05:00")
# HA helper deleted when timer finishes

# Named ephemeral
t = client.timer("my_cooldown")

# Persistent — stays in HA
t = client.timer("kitchen_timer", persistent=True)

Implementation

• Timer._handle_state_changed override schedules cleanup after all user listeners
• _auto_cleanup gracefully handles delete failures (logs + resets state)
• persistent=True without name raises ValueError
• 23 tests (179 total), timer.py at 100% coverage, overall 95.26%