pick-up sets the ticket to in_progress on accept (T-339)

When a ticket is handed to a live Claude pane (T-327), advance it to
in_progress on the receiving side of the bus — gated on acceptance, so a
pick-up with no live session stays a quiet no-op and never mutates state.
Only a not-yet-started ticket (backlog/ready) transitions, so re-picking
up a review/done ticket doesn't drag it backwards. On success it publishes
(builtin.tickets, changed) so the sidebar refreshes.

The handler logic moves into a testable applyTicketPickUp() seam; the
sidebar button now carries the current status in the pick-up payload.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -27,6 +27,12 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- **Picking up a ticket now starts it.** When a ticket is handed to a live
+  Claude pane via the sidebar pick-up button, it also transitions to
+  `in_progress` and the sidebar refreshes — but only on acceptance (a live
+  session took the prompt) and only from a not-yet-started status, so a
+  pick-up with no live pane stays a quiet no-op and a re-pick-up never drags a
+  `review`/`done` ticket backwards. (T-339)
 - **Clickable file references in the Claude conversation.** Workspace file paths
   mentioned by Claude — bare (`lib/app.dart`), with a line (`lib/app.dart:42`),
   backticked, or as markdown links — are now clickable and open in the editor,