refactor(provenance): extract agentSourceFields write-stamp helper (#143 review #5)

The agent write-stamp idiom — `...(isAgent ? { <source>: 'agent', <chat>: aiChatId } : {})`
— was hand-reimplemented at every REST write site, so each new path risked a
wrong literal or a forgotten aiChatId. Extract a single
`agentSourceFields(provenance, sourceKey, chatKey)` next to AuthProvenanceData and
call it at the 5 uniform spread sites:

- comment.service create  -> createdSource / aiChatId
- page.service create/update/orphan-move/move -> lastUpdatedSource / lastUpdatedAiChatId

Sites that must CLEAR the source on a non-agent action keep their own conditional
(comment un-resolve writes an explicit null), and the collab persistence path keeps
its sticky-window logic — both noted in the helper's doc.

Behavior-preserving (the helper returns the identical object/`{}`). Typecheck
clean; server comment/page/auth/collab suites 246 pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/core/comment/comment.service.ts

@@ -22,7 +22,10 @@ import {
   ICommentResolvedNotificationJob,
 } from '../../integrations/queue/constants/queue.interface';
 import { WsService } from '../../ws/ws.service';
-import { AuthProvenanceData } from '../../common/decorators/auth-provenance.decorator';
+import {
+  AuthProvenanceData,
+  agentSourceFields,
+} from '../../common/decorators/auth-provenance.decorator';
 
 @Injectable()
 export class CommentService {
@@ -60,7 +63,6 @@ export class CommentService {
   ) {
     const { page, workspaceId, user } = opts;
     const commentContent = JSON.parse(createCommentDto.content);
-    const isAgent = provenance?.actor === 'agent';
 
     if (createCommentDto.parentCommentId) {
       const parentComment = await this.commentRepo.findById(
@@ -87,9 +89,7 @@ export class CommentService {
       spaceId: page.spaceId,
       // Agent-edit provenance: the user stays creatorId; this only annotates the
       // source. Normal user requests leave the column default ('user').
-      ...(isAgent
-        ? { createdSource: 'agent', aiChatId: provenance.aiChatId }
-        : {}),
+      ...agentSourceFields(provenance, 'createdSource', 'aiChatId'),
     });
 
     if (createCommentDto.yjsSelection) {