feat(coach): document chats live in the Coach view and history

Asking the Coach about a document opened a separate, half-finished chat panel
that never appeared in Coach history. The bespoke drawer is gone; a document
conversation now opens the real Coach view, appears in the Coach list with a
document badge and a "Chatting about: <title>" note, and is read back through
the same history rail as any other thread.

The prompt-injection fence is unchanged and stays server-side. A document turn
is still sent through the hardened fenced endpoint (no tools, no health
snapshot, grounded in the document text only); the coach tool route continues
to fetch with documentId null, so a document-scoped conversation can never be
continued through the tool loop even from a hostile client. Only the surface
moved into the Coach; the boundary did not.

The relaxed conversation-list and detail filters drop only the documentId
predicate — userId scoping is unchanged, so a document thread is still visible
only to its owner. CoachConversationDTO gains documentId + documentTitle
(server-derived, title/filename only); no schema change (documentId exists
since 0230). OpenAPI regenerated.

src/lib/openapi/routes/coach.ts

@@ -508,6 +508,18 @@ const coachConversationSchema = z
       .datetime({ offset: true })
       .describe("Bumped on every appended message; the rail orders by this."),
     messageCount: z.number().int(),
+    documentId: z
+      .string()
+      .nullish()
+      .describe(
+        "When set, this thread is scoped to one stored document (a document chat, run on the hardened fenced endpoint). Null/absent for a normal health thread.",
+      ),
+    documentTitle: z
+      .string()
+      .nullish()
+      .describe(
+        "The owning document's resolved title (title, falling back to filename) for the scope badge. Null on a health thread or when the document has neither.",
+      ),
   })
   .meta({
     id: "CoachConversation",
@@ -548,6 +560,18 @@ const coachConversationDetailSchema = z
       .describe(
         "Rolling summary of turns elided past the history window; null when none is on file.",
       ),
+    documentId: z
+      .string()
+      .nullish()
+      .describe(
+        "When set, this thread is scoped to one stored document; the client routes its turns through the hardened fenced document endpoint, never the tool route.",
+      ),
+    documentTitle: z
+      .string()
+      .nullish()
+      .describe(
+        "The owning document's resolved title for the scope badge. Null on a health thread.",
+      ),
   })
   .meta({
     id: "CoachConversationDetail",