Add Nextcloud Talk bot integration (Inara)

- New routers/nextcloud_talk.py: webhook handler verifies incoming HMAC,
  calls LLM via BackgroundTasks, posts reply with correctly computed
  signature (random + message_text, not raw body)
- llm_client.py: read Claude OAuth token live from
  ~/.claude/.credentials.json to avoid stale systemd env tokens;
  strip conflicting ANTHROPIC_API_KEY
- config.py: add nextcloud_url, nextcloud_talk_bot_secret,
  nextcloud_talk_timeout settings
- main.py: register nextcloud_talk router, add logging setup
- docs/NEXTCLOUD_TALK_BOT.md: installation guide + HMAC signing gotcha

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

cortex/config.py

@@ -20,6 +20,12 @@ class Settings(BaseSettings):
     google_chat_timeout: int = 25
     # Backend forced for Google Chat — Claude is more reliable within the 25s deadline
     google_chat_backend: str = "claude"
+
+    # Nextcloud Talk bot
+    nextcloud_url: str = "https://cloud.dgrzone.com"
+    nextcloud_talk_bot_secret: str = ""   # set in .env
+    nextcloud_talk_timeout: int = 55
+
     host: str = "0.0.0.0"
     port: int = 8000