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/config.py

@@ -22,6 +22,11 @@ class Settings(BaseSettings):
     ae_account_id: str = ""       # x-account-id header
     ae_api_timeout: int = 15      # per-request timeout in seconds
 
+    # Agent identity — used in prompts, session logs, and memory distillation
+    # Override in .env for each instance (e.g. AGENT_NAME=Holly, USER_NAME=Holly)
+    agent_name: str = "Inara"
+    user_name: str = "Scott"
+
     inara_dir: Path = Path("../inara")
     sessions_dir: Path = Path("./data/sessions")
     default_model: str = "claude-sonnet-4-6"