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>

.env.default

@@ -1,6 +1,13 @@
 # Cortex .env reference — copy to .env and fill in values
 # DO NOT commit .env — it contains secrets
 
+# ── Agent identity ───────────────────────────────────────────────────────────
+# Each running instance has its own identity directory and name.
+# For a second instance (e.g. Holly), copy this file, change these values,
+# set a different PORT and INARA_DIR, and run a separate systemd unit.
+AGENT_NAME=Inara
+USER_NAME=Scott
+
 # ── Server ──────────────────────────────────────────────────────────────────
 HOST=0.0.0.0
 PORT=8000