fix(app): remove per-message Changed files section from chat output

[jeonghun-jj-lee]

Important

Problem — Two issues with the "Changed files" surfaces:

1. Noise. Every completed assistant turn appends a collapsible "N Changed files +X −Y" section at the bottom of the chat. This is redundant with the side-panel Files Changed tab (file tree, inline diffs, search) and noisy — a long session pushes dozens or hundreds of file rows into the conversation, displacing actual content.

2. Cross-session leak. File changes bleed across sessions. The snapshot system uses one shadow git repo per project (~/.local/share/opencode/snapshot/<project-id>/<worktree-hash>/), shared by all sessions. snapshot.track() captures the full working tree, so when sessions run concurrently, diffFull(step-start, step-finish) includes files changed by other sessions (or manual VS Code edits) between those two points.

Approach — Two changes:

• Overlay (amicode): Suppress the per-message DiffSummary timeline row in the row builder. Leave the rendering code, CSS, and i18n keys in place (dead but harmless). Guard the equivalent v1 session-turn-diffs section.
• Fork (harmoniqs/opencode, follow-up): Add agent-file filtering to summary.ts:computeDiff — mirror the agentFiles filter that Session.diff already applies in its primary path. This fixes the stored message.summary.diffs data (consumed by the side-panel's Fallback 1) and prevents the leak at the source.

Scope — in: suppress DiffSummary row in v2 timeline builder (rows.ts), guard v1 session-turn-diffs (session-turn.tsx). out: side-panel Files Changed tab, inline Edited-files tool groups, server-side summary.diffs computation, CSS/i18n cleanup (follow-up).

Assumptions — The v2 timeline layout is the active layout (dev channel, newLayoutDesigns default ON). The v1 path is guarded for completeness but is not the primary surface.

Acceptance Criteria

• Completed assistant turns no longer show the "N Changed files" collapsible section at the bottom of the chat message
• The side-panel Files Changed tab continues to show per-session file diffs correctly
• Inline "Edited files" tool groups (mid-message, per-tool-call) still render normally
• Existing tests pass (pnpm --filter amicode test)

Testing Decisions

Extend no new test suite — the overlay change is a deletion. Verify visually that the section is absent after an edit-heavy session and that the side panel still works. Existing overlay and parity tests cover the surrounding components.

Key Decisions

• Suppress, don't delete the renderer. The TimelineDiffSummaryRow component and its DiffSummary case in the timeline switch stay as dead code. This keeps the diff small and makes reversal trivial. A follow-up can clean up the dead code, CSS (session-turn-diffs selectors in session-turn.css), and orphaned i18n keys (ui.sessionTurn.diffs.*).
• Fork mirroring required. The overlay files are overwritten on each opencode:pin re-extraction. The same change must land in harmoniqs/opencode before the next pin bump, or the section will reappear.

Cross-session leak analysis

The snapshot diff pipeline has three layers with different isolation properties:

Surface	Data source	Leaks?	Why
Per-message "Changed files"	summary.ts:computeDiff (unfiltered snapshot diff)	Yes	No agent-file filter
Side panel (primary path)	Session.diff with agentFiles filter	Partial	Same-file concurrent edits share the cumulative diff
Side panel (Fallback 1)	Aggregated message.summary.diffs	Yes	Inherits per-message leak
Side panel (Fallback 2)	Tool part filediff metadata	No	Session-scoped
Touched-files endpoint	Tool part scan	No	Session-scoped
Client-side fallback	accumulateDiffs from loaded tool parts	No	Session-scoped

Root cause: summary.ts:computeDiff returns raw snapshot.diffFull(from, to) with no filtering by agent-touched files. The fork fix is to collect agentFiles from the turn's tool part filediff metadata (the same scan Session.diff does) and filter the result. This also fixes Fallback 1 by inheritance.

Constraints & Invariants

• Server-side summary.diffs computation must remain intact — the side panel and future consumers depend on it.
• The EditToolGroup component (inline "Edited files" groups during a live turn) is a separate code path and must not be affected.

Prior Art

• Timeline row builder: packages/app-bundle/overlay/packages/app/src/pages/session/timeline/rows.ts
• v1 session turn: packages/app-bundle/overlay/packages/session-ui/src/components/session-turn.tsx
• Side-panel review: packages/app-bundle/overlay/packages/app/src/pages/session/v2/review-panel-v2.tsx
• Server diff (with filter): packages/opencode/src/session/session.ts Session.diff
• Server diff (no filter): packages/opencode/src/session/summary.ts computeDiff

Notes

The per-message diffs section is an upstream opencode feature (introduced Feb 2026). The side-panel Files Changed tab is the preferred surface for reviewing file changes. The cross-session leak affects any project with concurrent sessions (spawned tabs, parallel agents) or external file edits during a turn.