Preserve session diff availability across refreshes and forks

[andagaev]

• Persist explicit empty, present, and unknown states while retaining known line totals when refreshes fail.
• Reset source-derived diff metadata on forks and refresh the new worktree state.
• Keep binary-only, metadata-only, and diagnostic diffs available through chat shortcuts and documentation.

[greptile-apps]

Greptile Summary

This PR introduces a three-state SessionDiffState enum (Unknown, Empty, Present) to preserve diff availability across session refreshes and forks, replacing the previous binary has/doesn't-have model.

• Persistence layer: Adds has_diff INTEGER column (NULL=Unknown, 0=Empty, 1=Present) via migration 061, with conservative backfill that leaves zero-line legacy rows as NULL (Unknown) to preserve access to binary/rename-only diffs. Fork reset and new session creation both initialize to NULL; mark_session_diff_unknown uses a no-op guard (AND has_diff IS NOT NULL) to avoid unnecessary WAL writes.
• In-memory state & UI: should_show_diff() returns true for both Unknown and Present, so the d: Diff shortcut remains visible when diff status is uncertain. Failed refreshes retain stale line totals but flip diff_state to Unknown, and all response-parser SessionStats constructions correctly supply diff_state: SessionDiffState::Unknown for token-only contexts.

Confidence Score: 5/5

Safe to merge — the three-state diff model is correctly implemented end-to-end with no blocking issues.

No P0 or P1 findings. The Unknown/Empty/Present model is consistently applied across the SQLite schema, fork SQL, DB read/write helpers, in-memory state, and UI visibility logic. Migration 061 conservatively handles binary/metadata-only diffs by leaving zero-line legacy rows as NULL rather than 0. The optimistic WHERE clause in update_session_diff_stats correctly uses SQLite's null-safe IS NOT operator, and mark_session_diff_unknown is a no-op when the state is already Unknown. Fork and new-session initialization both start at Unknown so the diff shortcut remains accessible until the first real refresh.

No files require special attention.

_{Reviews (12): Last reviewed commit: "Preserve session diff availability acros..." | Re-trigger Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud