Files Changed: trace one server-assessed non-Git external edit

Acceptance Criteria

• After edit permission succeeds and before mutation, an external edit or write captures an immutable server-local baseline for the active session.
• Denied permission creates no baseline record.
• A record becomes visible only after the final write and formatter complete successfully.
• The assessed-detail response is authenticated, no-store, and reports one settled changed assessment with a server-produced patch and line counts.
• Files Changed renders exactly one row for the sibling file after the detail response succeeds.
• The rendered patch comes from the assessed response, never the completed tool patch.
• While assessment is loading, failing, or absent, the app does not promote a tagged external tool patch.
• A second session has no assessment for the first session's sibling file.
• Existing in-worktree diff behavior and legacy consumers remain unchanged.

Testing Decisions

• Extend the existing session-scoped server-diff integration harness with one non-Git sibling fixture and a second-session isolation assertion.
• Extend edit and write tool tests to prove permission, baseline capture, mutation, and settlement ordering.
• Extend the app's existing diff-accumulation test surface so assessed external data wins and an unresolved assessment produces no external row.
• Extend the HTTP API coverage matrix for the assessed detail response.

Key Decisions

• This slice is a vertical tracer bullet, not the full external-file lifecycle.
• The baseline is process-local for this slice; later children own durable storage and cleanup.
• The assessed response is the only external truth the UI may render.
• The detail response uses an opaque session-owned reference; titles are never identity.

Constraints & Invariants

• Do not use the Git snapshot path as authority for the non-Git fixture.
• Do not modify extension file operations or watcher behavior.
• Do not expose baseline bytes in new metadata or the response outside the transient assessed patch.
• Keep the OpenCode fork as source of truth and synchronize the Amicode overlay afterward.

Prior Art

• Parent Files Changed: full session-lineage mutation ledger #972
• fix(app): trust server session diff for Files Changed panel #720, server authority for zero-net diffs
• Slice 1: Contamination fix — derive agent-file filter from tool filediff only #742, session-scoped ownership filtering
• Files Changed: show cross-project edits (merge server + tool-metadata diffs) #778, the rejected metadata-only external strategy

Source

Part of #972.

Notes

This child starts as hitl because the parent review is degraded. Its own deliberate review must pass before implementation begins.