fix: Fix stale context usage after compaction + usage calculation

[charlesvien]

Problem

1. Context usage calculation omitted output_tokens, under-reporting total token occupancy.
2. Context usage bar shows stale pre-compaction values until the next agent turn, making it look like context is still full after compaction.

Changes

1. Include output_tokens in context usage calculation (they become input on the next turn)
2. Emit a usage_update with used:0 immediately on compact_boundary
3. Reset local usage counter on compaction so subsequent turns start fresh

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 👈 (View in Graphite)
• 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
• 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.