feat: Add double-click to rename task title in session header

[charlesvien]

Problem

Task titles in the session view header are read-only, requiring users to go to the sidebar to rename. This makes renaming less discoverable as shown by every time I tell someone you can rename the task by double clicking the title they try this title and fail, not the one in the sidebar.

Changes

1. Add HeaderTitleEditor inline edit component with auto-focus and keyboard handling
2. Wire double-click on header title to toggle into edit mode
3. Persist rename with optimistic cache update, session store sync and API call (same pattern as sidebar)

How did you test this?

Manually

[charlesvien]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix: Fix stale context usage after compaction + usage calculation #1548
• fix: Show both local and remote repo name when they differ #1547
• fix: Queue messages during conversation compaction #1546
• fix: Prevent permission request from stealing focus across cells #1545
• feat: Add double-click to rename task title in session header #1544 👈 (View in Graphite)
• fix: Reduce unnecessary IPC calls causing UI unresponsiveness #1542
• fix: Move folder cleanup and orphaned worktree pruning to main process #1541
• fix: Show "edit" instead of "create" in permission prompt for existing files #1538
• fix: Remove double scrollbar from repo selector #1537
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.