workflow-fix: atomic post_event append (events.jsonl partial-line corruption)

Overview / Motivation

Auto-filed by the /daily problem sweep (2026-06-27) from a workflow-surface bug observed in the #653 autonomous session (transcript b8d3dbc3).

Goal

Make task_workflow.post_event crash-safe so a killed/OOM'd/cron-timed-out writer cannot leave a truncated JSON line that breaks every downstream reader.

Workflow gap

• Bug observed: post_event appends an event line via with path.open("a") under flock, but the write is NOT atomic against SIGKILL/OOM/cron-timeout mid-write. A watcher killed mid-append left a 129-byte partial JSON line (BAD line 374: Unterminated string) in tasks/.../653/events.jsonl, which then crashed every task.py view / list_events / dashboard read and stalled the /issue 653 resume for >2h.
• Why it is a workflow gap: events.jsonl is canonical task-workflow state; a single non-atomic append makes the entire task (and the dashboard) unreadable.
• Confidence (emitter): high (reproduced corruption + reader crash in-session).

Proposed change (refine in planning)

Two complementary fixes (planner picks one or both):

1. Atomic append: build the full json.dumps(event) + "\n" in memory and write it in a single os.write(fd, ...) (POSIX guarantees an atomic write for a buffer under PIPE_BUF; for larger lines, write to a temp + append, or O_APPEND single-syscall write) so a kill mid-write never leaves a partial line.
2. Tolerant reader: list_events / readers skip (and log) an unterminated trailing line instead of raising, so an already-corrupted file is still readable.

Add a concurrency/crash regression test under tests/test_task_workflow*.py (simulate a truncated trailing line; assert readers tolerate it; assert a concurrent append never interleaves a partial line).

Scope / surfaces

• Primary target: src/explore_persona_space/task_workflow.py (post_event, list_events / the events.jsonl reader).
• Add a regression test in tests/test_task_workflow*.py.

Constraints / invariants

• Preserve the existing flock on ~/.task-workflow/lock + single-commit semantics.
• Workflow-surface only — never experiment code.
• uv run pytest tests/test_task_workflow*.py passes; ruff on touched files passes.

Provenance

• workflow_fix_target: src/explore_persona_space/task_workflow.py
• fingerprint: postevent-atomic-append-corruption