Files Changed: model external mutation lifecycle and net-zero states

Acceptance Criteria

• Restoring a modified external file byte-for-byte to baseline yields unchanged and removes its Files Changed row.
• A file absent at first touch becomes added after committed creation and unchanged after committed deletion.
• A file present at first touch becomes deleted after committed deletion and unchanged when recreated with exact baseline bytes.
• A move to a missing destination yields independent source deleted and destination added assessments.
• A move to a pre-existing destination derives modified or unchanged from that destination's baseline and is never automatically added.
• A prepared first-touch mutation left after interruption is unavailable and produces no changed row.
• Aborting a first-touch reservation removes ownership; aborting a later reservation preserves the prior committed assessment.
• An expired, forged, wrong-session, wrong-reference, or stale-revision commit cannot replace the last committed expected state.

Testing Decisions

• Extend child 1's server integration harness with a table-driven create, delete, revert, abort, and paired-move matrix using real non-Git files.
• Extend tool mutation tests for registered external modification, creation, deletion, partial failure, and abort.
• Extend the assessed-detail UI coverage with visible zero-net row removal.
• Extend Sidebar operation tests only for prepare-before-mutate, commit-after-success, abort-after-failure, and paired move reservations.

Key Decisions

• A reservation group represents one logical mutation; a move reserves source and destination together while keeping their baselines distinct.
• Commit records only server-verified present or absent endpoint state.
• Filesystem failure aborts attribution rather than inventing a partial diff.
• Watcher events remain invalidation only and never infer lifecycle state.

Constraints & Invariants

• Do not add another baseline store, canonical resolver, or browser identity scheme.
• Do not use client titles, watcher status, tool patches, or rename maps as lifecycle truth.
• Do not expand undeclared shell attribution.
• Keep parent Files Changed: full session-lineage mutation ledger #972's privacy and legacy compatibility boundaries intact.

Prior Art

• Parent Files Changed: full session-lineage mutation ledger #972 lifecycle matrix
• fix(app): trust server session diff for Files Changed panel #720, zero-net server authority
• Slice 1: Contamination fix — derive agent-file filter from tool filediff only #742, session-scoped ownership filtering
• Files Changed: per-session file watcher for reliable external-change refresh #845, watcher invalidation semantics

Source

Part of #972. Blocked by #973.

Notes

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