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/infrastructure/src/lib/graphql/fragments.graphql

@@ -121,6 +121,9 @@ fragment IssueSummaryFields on Issue {
 fragment IssueFields on Issue {
   ...IssueSummaryFields
   pinned
+  decisions {
+    ...CommentFields
+  }
   description
   figmaUrl
   createdAt
@@ -178,6 +181,10 @@ fragment CommentFields on Comment {
       ...UserRefFields
     }
   }
+  decision
+  decidedBy {
+    ...UserRefFields
+  }
   createdAt
   editedAt
   author {