Tracking B2: per-task change journal with torn-tail repair

[easonLiangWorldedtech]

Tracking issue for the B2 PR (branch feat/change-journal-b2; PR to be opened after review).

Part of the file-write safety series — plan: easonLiangWorldedtech/Zoo-Code#33, epic: #1375. Item: B2 — Per-task change journal. Stacked on #1395 (S3) + the B1 per-write checkpoint PR; to be rebased onto main once both land.

Scope

• Append-only changes.jsonl under the task dir (<globalStorage>/tasks/<taskId>/changes.jsonl): one entry per file write — path (relative), operation, the B1 checkpoint id, diff stats, timestamp.
• The journal append chains onto the B1 per-write checkpoint so it records the real checkpoint hash; the whole tail stays fire-and-forget (no write-path latency, no floating promises).
• apply_patch records one entry per file, all referencing the single checkpoint id.
• Torn-tail repair on load: an incomplete final line is discarded and the rest parsed; load never throws on a torn tail (crash mid-append leaves a valid audit list).

Acceptance

• One journal entry per successful write referencing the resolved checkpoint id; none for rejected writes or the task-start baseline
• Load: clean tail, torn tail, empty file, missing file, and a corrupted middle line all resolve without throwing
• Journal is the single per-task audit list and survives a crash mid-write
• CI green (ubuntu unit-test gate), 100% codecov patch coverage

Status

• PR: to be opened