Add yellow highlight animation for newly created beads in Plan mode

[newhook]

Summary

Implements visual feedback for newly created beads in the Plan mode TUI. When a new bead is created (via bd create or other means), it now appears with yellow text highlighting that automatically fades after 5 seconds, making it easy to spot new items in the tree.

Changes

• cmd/tui_plan.go: Added tracking for newly created beads using a newBeads map that stores bead IDs with their creation timestamps. Implemented detection logic that compares incoming beads against existing items to identify new ones. Added timer-based message system (newBeadExpireMsg) to automatically remove the highlight after 5 seconds.

• cmd/tui_plan_render.go: Added rendering logic to apply the yellow text style to beads marked as new in the newBeads map.

• cmd/tui_shared.go: Added new tuiNewBeadStyle using lipgloss color 226 (yellow) for the animation highlight.

Implementation Details

• New beads are detected by comparing the refreshed bead list against the existing items
• Each new bead triggers a 5-second timer that fires a message to remove the highlight
• The animation only triggers when there are existing beads (avoids highlighting everything on initial load)
• Uses the existing tea.Tick mechanism for timer-based expiration

Issues Resolved

• ✅ ac-f6og: New issues should be animated in the tree when created
  • ✅ ac-f6og.1: Track newly created beads with timestamp
  • ✅ ac-f6og.2: Detect new beads when refreshing data
  • ✅ ac-f6og.3: Add yellow text style for new items
  • ✅ ac-f6og.4: Implement timer to remove animation after 5 seconds
  • ✅ ac-f6og.5: Test animation behavior with bead creation

Testing

• Create a new bead using bd create while Plan mode is open
• Observe the new bead appears with yellow text
• After 5 seconds, the yellow highlighting should automatically disappear
• The animation should not trigger on initial TUI load

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