Add an activity timeline to the preview pane

[jongio]

I want the preview pane to tell the story of a session at a glance, not just show chat turns and small metadata blocks. A timeline mode would make long sessions easier to scan before I resume them.

flowchart TD
  A[Selected session] --> B[Preview pane]
  B --> C[Timeline mode]
  C --> D[Turns]
  C --> E[Checkpoints]
  C --> F[Refs]
  C --> G[Files]
  C --> H[Tool events]
  H --> I[Scrollable rows]

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Problem

Long sessions can span many turns, checkpoints, file touches, and refs. Today I can read the preview pane, but I still have to piece together when important events happened and why a session is worth resuming.

Proposed solution

Add a timeline view to internal/tui/components/preview.go that can be toggled from the preview pane with a new key in internal/tui/keys.go. Build the timeline from existing session data in internal/data/store.go and event parsing in internal/copilot/events.go, then render timestamped rows for turns, checkpoints, refs, files, and notable tool activity. Keep the existing chat preview as the default and add tests beside internal/tui/components/preview_test.go and internal/data/store_test.go.

User impact

I can scan a long session in seconds, spot the key turn or checkpoint, and decide whether to resume, hide, favorite, or compare it with another session.

Acceptance criteria

• Add a preview timeline mode that does not replace the current chat preview.
• Show timestamped rows for turns, checkpoints, refs, files, and notable tool activity when data exists.
• Add a keybinding and help text for toggling the timeline mode.
• Keep preview scrolling and preview position behavior working in timeline mode.
• Cover timeline row building and rendering with Go tests.

Complexity

M