Tracking #1384: L2 chat-diff as default approval path #1389

Tracking issue for upstream PR #1384 (branch feat/l2-chat-diff-default).

Part of the file-write safety series — plan: easonLiangWorldedtech/Zoo-Code#33, epic: #1375. Item: L2 — Chat-diff as the default approval path.

Scope

• PREVENT_FOCUS_DISRUPTION experiment default flipped false → true in src/shared/experiments.ts: approval happens in chat, writes are saved directly, no diff-editor open/refocus. Toggle kept as escape hatch; storage key unchanged, so saved values are preserved.
• All five write tools + ApplyDiff already route through experiments.isEnabled(state?.experiments ?? {}, ...) — the flip applies automatically, no production call-site changes.
• Tests: default assertions flipped; new default-path behavioral test (no stored value → saveDirectly, no open/saveChanges; stored false → legacy diff-editor path); tool-spec base mocks pin the legacy path explicitly; the openFile=false case now also asserts the in-memory-only document load (focus not stolen).

Acceptance

• Default approval no longer opens/refocuses the diff editor
• No setting rename, no storage migration
• CI green (ubuntu unit-test gate)

Status

• PR: OPEN, MERGEABLE — awaiting CI