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-holly.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Cortex / Holly LLM Gateway
+After=network.target
+
+[Service]
+Type=simple
+User=scott
+WorkingDirectory=/home/scott/agents_sync/projects/Cortex_and_Inara_dev/cortex
+EnvironmentFile=/home/scott/agents_sync/projects/Cortex_and_Inara_dev/cortex/.env.holly
+ExecStart=/home/scott/agents_sync/projects/Cortex_and_Inara_dev/cortex/.venv/bin/uvicorn main:app --host 0.0.0.0 --port 8001
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target