Tracking B1: per-write checkpoints, task-start baseline, and perWriteCheckpoints setting

[easonLiangWorldedtech]

Tracking issue for the B1 PR (branch feat/per-write-checkpoints-b1; PR to be opened after review).

Part of the file-write safety series — plan: easonLiangWorldedtech/Zoo-Code#33, epic: #1375. Item: B1 — Per-write checkpoint. Stacked on #1395 (S3 atomic publish); to be rebased onto main once #1395 lands.

Scope

• Every successful write_to_file / edit_file / apply_patch records a shadow-git checkpoint after the write succeeds (one per write; apply_patch = one checkpoint after the whole patch), in addition to the existing pre-write and user-message checkpoints. The checkpoint reuses the existing shadow git at <globalStorage>/tasks/<taskId>/checkpoints and the existing checkpoint_saved plumbing — no new say/webview message types.
• Task start records one O(1) baseline checkpoint (allowEmpty, suppressed, no worktree), exactly once per task.
• The per-write checkpoint and the baseline are non-blocking (fire-and-forget, crash-safe) so the write path keeps its ≈0 added latency.
• New setting perWriteCheckpoints (boolean, default true) — the master switch for the B cluster, gating both behaviors; ships with the full persisted-setting round trip (types, ExtensionState, ClineProvider getState/getStateToPostToWebview, webview SettingsView cachedState save flow, CheckpointSettings UI, all locales).

Acceptance

• One checkpoint per successful write (mock-service assertions per tool); none on rejected or failed writes
• Task-start baseline recorded exactly once per task instance, cheap (no worktree)
• Rollback index: the checkpoint id is surfaced through the existing checkpoint_saved / currentCheckpointUpdated plumbing
• Setting round trip: default true; stored true/false/unset all resolve correctly in getStateToPostToWebview(); UI save via cachedState; import/export round trip
• CI green (ubuntu unit-test gate), 100% codecov patch coverage

Status

• PR: to be opened