action_log: Fix race condition when committing changes

[bennetbo]

Sometimes the action log would not auto-accept agent edits when commiting.

Gpt-5.4 identified this race condition:

This fixes a race where keep_committed_edits could run after head_commit changed but before the new git base text had been applied, leaving committed agent edits marked as unreviewed; ActionLog now waits for an explicit BufferDiffEvent::BaseTextChanged instead of inferring readiness from generic DiffChanged activity, so it only accepts edits after the diff base itself is actually updated.

• ReloadGitState updates head_commit before ReloadBufferDiffBases finishes loading and applying the new HEAD text.
• In that gap, an unrelated DiffChanged can fire from a normal diff recalculation.
• The old logic treated that event as the commit signal and ran keep_committed_edits too early.
• keep_committed_edits then read stale diff base text, so it failed to match the committed agent edits.
• When the real base-text update arrived later, the HEAD had already been overwritten (old_head), and the edits stayed unreviewed.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

• Fixed an issue where committing agent written code would sometimes not mark edits as accepted