fix: sidebar updates for /undo and compacting

[flatsponge]

Adresses:
#5760 #4548

two very similar issues, so i batch fixed them. let me know if I should split it instead.

Issue:
compacting would not update the sidebar immediately after successful compaction and /undo does also not update Modified Files.

Cause:
Issue 1:
The /compact command triggers session.compacted event but session.compacted was NOT handled in
sync.tsx so the sidebar never updated

Issue 2:
/undo Command does not emit a session.diff event.
Only session.updated is published via Session.update causing the sidebar's Modified Files to never get updated.

Fix:
Issue 1:
Add handler for session.compacted

Issue 2:
After /revert emit a session.diff event with an empty diff array to clear the Modified Files list in the sidebar