fix(agent): freeze elapsed timer after completion

[defia]

Summary

• preserve the last agent hook timestamp in pane summaries
• freeze elapsed time for completed, failed, and awaiting-reply states while keeping active states live
• add regression coverage for completed and active timer behavior

Root cause

Codex emitted the Stop hook correctly, but the pane-summary TimelineView always calculated elapsed time against the current clock. Completed rows therefore kept counting after their status changed.

Testing

• xcodebuild ... -only-testing:GlintTests test
• 312 tests passed, 0 failures