Replace 'Files Changed' with session-scoped agent changes

[jeonghun-jj-lee]

Important

Problem: The Files Changed panel currently shows git working-tree diffs — all uncommitted changes regardless of source. This is noise when the user cares about what this session's agent actually wrote.

Approach: Replace the git/branch/turn mode system entirely with a single session-scoped view. Compute a net diff (session-start snapshot vs current state) filtered to only files the agent touched (via PatchParts). Reuse the existing snapshot infrastructure and file tree UI.

Scope: Backend: revive Session.diff with snapshot-based net-diff logic. Frontend: remove VCS mode machinery, wire the panel to the session diff endpoint. No new dependencies.

Assumptions: Snapshot tree hashes survive for the session's lifetime (GC is 7 days; sessions rarely outlive that). Bash-authored files are captured via PatchParts (snapshot-based, not tool-metadata-based).

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Acceptance Criteria

• The Files Changed panel shows only files written by the agent during the current session
• The diff shown is the net diff (state at session start vs current state), not per-turn patches
• A file tree is rendered on the left with A/D/M badges; selecting a file shows its diff on the right
• No git/branch/turn mode selector exists in the UI
• A session with no agent file changes shows an empty state ("No changes yet")
• Files touched via Bash (e.g. sed, mv) are included (PatchPart captures them)
• Files the agent edited then externally reverted show no diff (filtered out when additions + deletions === 0)
• Sessions older than snapshot GC (7 days) gracefully show empty state

Key Decisions

Decision	Choice	Rationale
Mode system	Remove entirely	User only wants session context; git/branch are noise
File set source	Union of PatchPart.files across session	Captures all agent-authored files including Bash writes
Diff computation	snapshot.diffFull(firstStepStart, current)	Gives true net diff; reuses existing infra
Filtering	Intersect diffFull result with PatchPart file set	Excludes external changes that happened to land in the snapshot window
Empty/stale session	Return [], render empty state	Graceful degradation, no error

Implementation Sketch

Backend (packages/opencode/src/session/)

1. Session.diff — replace the stub with:

   • Query all messages + parts for the session
   • Extract first step-start hash (session-start ref)
   • Union all PatchPart.files (agent-touched set)
   • snapshot.track() for current state
   • snapshot.diffFull(from, to) filtered to agent-touched files
   • Filter out zero-change entries

2. HTTP handler — GET /session/:sessionID/diff already wired; just returns the revived Session.diff result

Frontend (packages/app/src/pages/session.tsx + side panel)

1. Remove reviewMode signal and ChangeMode type
2. Remove vcsQuery (the sdk().api.vcs.diff(...) call)
3. Replace reviewDiffs() with a single query to sdk().api.session.diff(sessionID)
4. Re-fetch on file.watcher.updated events (same trigger as before)
5. File tree + diff viewer stay as-is, fed by the new data source
6. Empty state when diffs.length === 0

TUI (packages/tui/)

• Remove mode selector from diff-viewer
• Point at session diff endpoint

Constraints & Invariants

• Snapshot tree objects must exist for the session's first turn (GC = 7 days)
• PatchPart is the canonical source of "which files the agent touched" — no alternative tracking needed
• The file tree component (file-tree-v2.tsx) and diff viewer are untouched — only their data source changes