feat(voice): unify Discord voice↔text session (squash of voice/text-unify)

Voice utterances and text messages on the same Discord channel now share
one Claude session, and Echo's voice replies are mirrored back into the
text channel. Replaces the old voice:<id> session-key split.

Changes:
- src/adapters/_text_chunks.py: new leaf module for split_message
  (used by both discord_bot and voice pipeline)
- src/router.py: drop voice: prefix from session_key; add [voice] marker;
  strip leading [speaker:/[voice] tokens from user input (anti-jailbreak);
  remove dead double-clear of voice: key
- src/claude_session.py: include personality/VOICE_MODE.md unconditionally
  (rules become per-turn-aware via [speaker:] prefix instead of session flag)
- src/voice/pipeline.py: VoiceSession splits text_channel_id +
  voice_channel_id; resolve text channel per-send (no stale refs); mirror
  Echo's reply text into the text channel after route_message returns
- src/adapters/discord_voice.py: /voice join passes both channel ids
- src/adapters/discord_bot.py: import split_message from leaf module
- personality/VOICE_MODE.md: rewrite as per-turn dynamic rules;
  add synthesis instructions for text turns after voice turns

Tests:
- tests/test_router.py: 4 new cases (plain channel_id, anti-jailbreak,
  text-adapter regression, no-double-clear)
- tests/test_pipeline_mirror.py: new — Echo reply mirror chunking,
  empty guard, mirror_enabled=False, send-raises resilience
- tests/test_voice_session_channel_ids.py: new — split-attr contract
  + metrics payload schema
- tests/test_voice_session_cleanup.py: update for new kwargs

Plan: /home/moltbot/.claude/plans/vreau-ca-tot-textul-greedy-rivest.md

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_voice_session_cleanup.py

@@ -76,10 +76,13 @@ def _make_session(
     record_enabled: bool = True,
 ) -> VoiceSession:
     jsonl = tmp_path / ("transcripts.jsonl" if record_enabled else "noop.jsonl")
+    # mock_text_channel kept resolvable via bot.get_channel for any future
+    # send invocations; cleanup tests don't exercise mirror, just attribute.
+    mock_bot.get_channel = MagicMock(return_value=mock_text_channel)
     return VoiceSession(
-        channel_id=1001,
+        text_channel_id=1001,
+        voice_channel_id=2002,
         guild_id=42,
-        text_channel=mock_text_channel,
         voice_client=mock_voice_client,
         bot=mock_bot,
         ttsq=mock_ttsq,