fix(opencode): improve message filtering for summary update during revert and unrevert

[OpeOginni]

What does this PR do?

Fixes #13488

• Fix revert diff/summary calculation to use the messages that remain after a revert
• Recompute diff/summary on unrevert so the UI matches the restored workspace
• Preserve the stacked revert snapshot baseline behavior

Problem
The revert summary was being built from the reverted message range instead of the remaining active messages. That caused the UI to show files that had been reverted, rather than the files still represented by the session.
unrevert also restored the snapshot on disk without recomputing the session diff/summary, which could leave stale summary data behind.

Fix
On revert, the session diff is now recomputed from the remaining messages:

• full message revert: messages before the reverted message
• part revert: messages up to and including the reverted message

On unrevert, after restoring the saved snapshot, the full session diff and summary are recomputed and written back.

How did you verify your code works?

Screen.Recording.2026-02-13.at.15.56.50.mov

[rekram1-node]

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