Fix extra button to only work when OFF-hook and add greeting delay

Breaking Behavior Change:
- Extra button NOW only works when handset is OFF-hook (during call)
- Previous: worked when on-hook (idle) - REVERSED
- Reason: Button is for guests to trigger during their call/recording

New Feature - Greeting Delay:
- Add greeting_delay_seconds to config.json (0-10 seconds)
- Delays greeting playback after handset pickup
- Gives guests time to position phone comfortably
- Default: 0 (plays immediately, backward compatible)

Extra Button Logic:
- Only responds when phone_status == "off_hook"
- Ignored when phone is on-hook (idle)
- Can be pressed during greeting or recording
- Plays entire sound without interruption
- Useful for: "Press button to hear special message"

Greeting Delay Use Cases:
- Wedding: Give guests moment to settle in
- Events: Time to position phone comfortably
- Accessibility: Extra time for elderly guests
- Professional: Pause before message delivery

Configuration:
{
  "system": {
    "greeting_delay_seconds": 2,  // 2 second pause
    "extra_button_sound": "surprise.wav"
  }
}

Console Output:
- "Extra button ignored - phone is on hook" (when idle)
- "Waiting X seconds before greeting..." (delay active)
- "=== Extra button pressed ===" (during call)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

config.example.json

@@ -26,6 +26,7 @@
   "system": {
     "active_greeting": "dialtone.wav",
     "extra_button_sound": "button_sound.wav",
+    "greeting_delay_seconds": 0,
     "volume": 70
   }
 }