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>

src/adapters/discord_bot.py

@@ -28,6 +28,7 @@ from src.router import (
     planning_cancel,
     start_planning_session,
 )
+from src.adapters._text_chunks import split_message
 from src.adapters.discord_views import (
     RalphRootView,
     PlanningActiveView,
@@ -80,28 +81,6 @@ def _channel_alias_for_id(channel_id: str) -> str | None:
     return None
 
 
-# --- Message splitting helper ---
-
-
-def split_message(text: str, limit: int = 2000) -> list[str]:
-    """Split text into chunks that fit Discord's message limit."""
-    if len(text) <= limit:
-        return [text]
-
-    chunks = []
-    while text:
-        if len(text) <= limit:
-            chunks.append(text)
-            break
-        # Find last newline before limit
-        split_at = text.rfind('\n', 0, limit)
-        if split_at == -1:
-            split_at = limit
-        chunks.append(text[:split_at])
-        text = text[split_at:].lstrip('\n')
-    return chunks
-
-
 # --- Factory ---