plugins: merge the user Plugins page into per-plugin sidebar pages

The generic per-user #plugins page is gone: a plugin with per-user
settings hosts them in its own web_pages() sidebar page instead
(Telegram's pairing page is new; Honcho's opt-in page already existed).
The admin catalog moves from #plugin-catalog to #plugins (old hash
redirected), and user_config_schema is removed from the Plugin trait,
the API DTOs and both plugins — the my-config endpoint, the
plugin_user_configs store and the update_user_config hook stay, now
driven by each plugin's own page fragment.

docs/plugins/honcho.md

@@ -17,7 +17,7 @@ Streams a user's completed chat turns to an external [Honcho](https://honcho.dev
 
 ## Enabling & configuring (admin)
 
-1. Plugin catalog → **Honcho Memory** → enable, then **Configure** (or its own admin page, once enabled: sidebar → Honcho).
+1. Plugins page → **Honcho Memory** → enable, then **Configure** (or its own admin page, once enabled: sidebar → Honcho).
 2. Fields:
    - **`base_url`** (default `http://localhost:8000`) — the Honcho server's URL.
    - **`api_key`** — optional, only if the server requires auth.