feat: per-user channel config for Google Chat and Nextcloud Talk

- New endpoints: POST /channels/google-chat/{username} and /webhook/nextcloud/{username}
- Channel secrets/config live in home/{username}/channels.json (gitignored)
- auth_utils: get_user_channels() helper reads channels.json
- Both routers load persona, audience/secret, backend, timeout per user;
  set_context() wires the correct persona before building the system prompt
- Removed server-level channel settings from config.py and .env —
  no user gets a channel until they create their own channels.json
- .gitignore: home/**/channels.json added

To migrate: update Google Chat Add-on webhook URL to /channels/google-chat/{username}
and re-register NC Talk bot at /webhook/nextcloud/{username}

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

cortex/auth_utils.py

@@ -195,3 +195,22 @@ def consume_invite(username: str) -> None:
             path.write_text(json.dumps(data) + "\n")
         except Exception:
             pass
+
+
+# ---------------------------------------------------------------------------
+# Per-user channel config
+# ---------------------------------------------------------------------------
+
+def _channels_path(username: str) -> Path:
+    return settings.home_root() / username / "channels.json"
+
+
+def get_user_channels(username: str) -> dict:
+    """Return the parsed channels.json for a user, or {} if not found."""
+    path = _channels_path(username)
+    if not path.exists():
+        return {}
+    try:
+        return json.loads(path.read_text())
+    except Exception:
+        return {}