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.

crates/plugin-honcho/src/lib.rs

@@ -829,27 +829,6 @@ impl core_api::plugin::Plugin for HonchoPlugin {
         })
     }
 
-    /// Per-user opt-in. Honcho stores conversations in cleartext on an external
-    /// server, so a user must knowingly enable it. A plain boolean — no secrets —
-    /// so the admin-readable `plugin_user_configs` store is an honest home. The
-    /// default `update_user_config` (store the blob) is exactly right; no override.
-    fn user_config_schema(&self) -> Value {
-        json!({
-            "type": "object",
-            "properties": {
-                "enabled": {
-                    "type":        "boolean",
-                    "title":       "Enable long-term memory",
-                    "description": "Let the assistant remember you across sessions. \
-                                    Your messages will be stored in cleartext on the \
-                                    Honcho memory server, outside your encrypted \
-                                    database. Off unless you turn it on.",
-                    "default":     false
-                }
-            }
-        })
-    }
-
     /// Two dedicated pages served from this plugin's own router (`web/*.js`):
     /// an **admin** config page (connection + a connectivity test) and a
     /// **user** opt-in page (the per-user consent to long-term memory). The