feat: comments marked as decisions, on top of the ticket and searchable

Whoever can change the ticket flags a comment as a decision (new event
CommentDecisionMarkedEvent on the comment). The ticket page lists its
decisions above the description, with a link to the comment in the
conversation (`?comment=<id>` scrolls to it and points at it), and the
search palette finds them by their text across my projects.

API: markDecision / PUT-DELETE /api/comments/{id}/decision; Issue.decisions
and GET /api/issues/{issue}/decisions; decisions(projectId?, search) and
GET /api/projects/{project}/decisions, GET /api/me/decisions. MCP:
mark_decision, search_decisions, and get_issue includes them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/web/tracker/domain/src/lib/models/models.ts

@@ -192,6 +192,8 @@ export interface Issue extends IssueSummary {
   watchers: UserRef[];
   /** Whether I keep it pinned (home page and search). */
   pinned: boolean;
+  /** Its comments marked as decisions, oldest first. */
+  decisions: Comment[];
   checklist: ChecklistItem[];
   /** Commits, branches and pull requests that mention it. */
   devLinks: DevLink[];
@@ -326,10 +328,20 @@ export interface Comment {
   deleted: boolean;
   /** 👍… and who chose each (only the ones somebody chose). */
   reactions: { emoji: string; users: UserRef[] }[];
+  /** Marked as a decision of the ticket: shown above it and searchable. */
+  decision: boolean;
+  /** Who marked it as a decision (null when it is not one). */
+  decidedBy: UserRef | null;
   createdAt: string;
   editedAt: string | null;
 }
 
+/** A decision found by its text, with its ticket. */
+export interface Decision {
+  comment: Comment;
+  issue: { id: string; key: string; title: string };
+}
+
 export interface SprintProgress {
   total: number;
   /** Done or cancelled. */