feat: multi-instance support — agent_name and user_name configurable

All hardcoded "Inara"/"Scott" strings replaced with settings.agent_name
and settings.user_name, read from .env at startup:

- config.py: AGENT_NAME and USER_NAME settings (defaults: Inara / Scott)
- llm_client.py: conversation labels in prompt builder
- session_logger.py: **Name:** labels in session log markdown
- memory_distiller.py: distillation system prompts (mid + long)
- routers/nextcloud_talk.py: @mention prefix strip
- routers/google_chat.py: greeting message

Second instance scaffolding:
- holly/: identity directory with placeholder files (USER_NAME=Holly,
  AGENT_NAME to be chosen by Holly)
- cortex/.env.holly: config for Holly's instance on port 8001
- cortex-holly.service: systemd unit for the second instance

No behavioural change to the Inara/Scott instance — defaults unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/session_logger.py

@@ -17,6 +17,6 @@ def log_turn(session_id: str, user_msg: str, assistant_msg: str) -> None:
             f.write(f"# Session Log — {today}\n")
         f.write(
             f"\n### [{timestamp}] `{session_id}`\n"
-            f"**Scott:** {user_msg}\n\n"
-            f"**Inara:** {assistant_msg}\n"
+            f"**{settings.user_name}:** {user_msg}\n\n"
+            f"**{settings.agent_name}:** {assistant_msg}\n"
         )