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]

• 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

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[charlesvien]

Merge activity

• Apr 8, 11:25 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Apr 9, 12:19 AM UTC: Graphite rebased this pull request as part of a merge.
• Apr 9, 12:30 AM UTC: @charlesvien merged this pull request with Graphite.