daily-fix: task-workflow API atomicity (git-mv, post-marker, view)

Overview / Motivation

Auto-filed by the /daily three-route problem sweep (2026-06-30), route 2 (behavior/logic change → independent review pipeline).

Goal

(a) Make the status-transition move whole-task-dir + REGISTRY update atomic under one flock+commit, rolling back on partial failure before touching REGISTRY; (b) in get_task/_read_body raise a typed error naming the stale path + the task.py audit remedy; (c) make post-marker's events.jsonl append atomic (temp+rename, or hold flock across the full write) so a mid-write kill cannot leave a partial line; (d) wrap cmd_view's stdout print in a SIGPIPE-safe guard.

Workflow gap

• Bug observed: Task #722 was genuinely corrupted this day: a non-atomic status-transition git mv split the task across two folders (events.jsonl+artifacts moved, body.md+plans did not) and left REGISTRY with no #722 entry; get_task/_read_body then crash with a bare FileNotFoundError instead of a typed 'registry stale, run task.py audit' error; a Bash-timeout-killed post-marker left a partial/corrupt events.jsonl append needing manual byte-offset truncation; and task.py view --json piped to a consumer that exits early tracebacks with BrokenPipeError.
• Evidence: living_docs check reports #696/#722/#750/#764/#766 registry/filesystem inconsistent today. Sources: /daily miners (batches 04/05/06/08).
• Confidence (emitter): medium

Scope / surfaces

• Primary target: src/explore_persona_space/task_workflow.py
• Grep the workflow surface for the pattern before editing and update every hit; list them in the plan.

Constraints / invariants

• Workflow-surface / code fix per the target; keep ruff + workflow_lint + the relevant tests green.
• The planner may deflect with a reasoned no-change report if the gap is already closed on main.

Provenance

• workflow_fix_target: src/explore_persona_space/task_workflow.py
• fingerprint: ba9fdef5836a
• source: /daily route-2 (2026-06-30)