fix(tui): restore modified files sidebar

[letanphuc]

Issue for this PR

Closes #30877

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The TUI Modified Files section has been empty since session-level diffs were disabled in #30127. The sidebar still requests a session diff without a message ID, but that request currently returns an empty list.

This change tracks the files changed by write, edit, and apply_patch. Before the first edit, the original file state is saved in private session storage. The session diff endpoint compares only those tracked files with their current contents, so generated build output and unrelated working-tree changes are not shown. Files restored to their original state, including files created and later deleted, disappear from the list.

The TUI refreshes this lazy diff after an assistant turn completes. Existing sessions continue to use snapshot diffs or legacy tool metadata when private baselines are unavailable.

Baseline contents are removed from tool metadata before tool parts are persisted or emitted, because tool metadata is exposed by session APIs and sharing.

How did you verify your code works?

• Ran bun typecheck from packages/opencode and packages/tui.
• Ran 64 focused opencode tests covering session diffs, processor behavior, write, and edit.
• Ran 9 TUI sync and hydration tests.
• The pre-push hook ran the repository-wide typecheck: 30 tasks passed.
• Tested non-Git projects, missing .gitignore, generated build files, relative paths, and create-then-delete behavior.

Screenshots / recordings

No visual styling changed. The existing Modified Files section is populated again after agent edits.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found two potentially related PRs that address similar issues:

1. PR fix(opencode): restore session diff summary #37542: fix(opencode): restore session diff summary

   • fix(opencode): restore session diff summary #37542
   • Related because it addresses restoring session-level diffs, which is closely tied to the modified files sidebar functionality in this PR.

2. PR fix(session): restore session summary from per-turn diffs (#30877) #33444: fix(session): restore session summary from per-turn diffs ([Bug] v1.16.0: TUI sidebar "Modified Files" section completely hidden after path truncation fix #30877)

   • fix(session): restore session summary from per-turn diffs (#30877) #33444
   • Related because it directly addresses issue [Bug] v1.16.0: TUI sidebar "Modified Files" section completely hidden after path truncation fix #30877 (the same issue closed by the current PR) and focuses on restoring session summary from per-turn diffs rather than full snapshots.

Both PRs appear to be addressing the same root problem of restoring diff and summary functionality after #30127 removed eager session-level diff generation. The current PR (37613) specifically focuses on the TUI sidebar restoration with per-file baseline tracking, while #37542 and #33444 address related session-level diff restoration work.

[github-actions]

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