docs(spec): drop residual OpenRouter mentions

Three references slipped through the HF Inference migration:
the system diagram, the API route table (/api/chat description)
and the agent sequence diagram all still named OpenRouter as the
LLM backend. Replace them with HF Inference (Providers) to match
the actual implementation in stream-handler.ts.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/SPECIFICATION.md

@@ -22,7 +22,7 @@ graph TB
     Express["Express HTTP"]
     Hocuspocus["Hocuspocus<br/>WebSocket"]
     Publisher["Publisher Pipeline<br/>HTML + PDF"]
-    Agent["AI Agent<br/>OpenRouter"]
+    Agent["AI Agent<br/>HF Inference"]
     Auth["HF OAuth"]
   end
 
@@ -72,7 +72,7 @@ All types are concurrently editable by multiple users and persist to `data/defau
 | `GET` | `/oauth/authorize` | Public | Redirect to HF OAuth |
 | `GET` | `/auth/callback` | Public (CSRF state) | Exchange code, set cookie, redirect to `/editor` |
 | `GET` | `/api/auth/status` | Cookie | Return `{ authenticated, canEdit, user }` |
-| `POST` | `/api/chat` | **None** | Stream AI agent responses (OpenRouter) |
+| `POST` | `/api/chat` | OAuth (optional) | Stream AI agent responses (HF Inference Providers) |
 | `POST` | `/api/publish` | OAuth (canEdit) | Run publish pipeline, generate HTML/PDF |
 | `POST` | `/api/admin/reset-document` | OAuth (canEdit) | Delete local `.yjs`, close connections |
 | `POST` | `/api/upload` | None (uses cookie for HF) | Upload image (multipart, max 10MB) |
@@ -378,7 +378,7 @@ sequenceDiagram
   participant User as Chat Panel
   participant Hook as useAgentChat
   participant API as POST /api/chat
-  participant LLM as OpenRouter
+  participant LLM as HF Inference
 
   User->>Hook: sendMessage(text)
   Hook->>Hook: Build context (doc, selection, frontmatter)