fix: surface silent task loss from auto-clear and disk failures

[kylesnowschwartz]

Disclosure

This patch was AI-generated (Claude) at my request, motivated by a few silent-failure modes I noticed while using pi-tasks in real work — most painfully, completed-tool events for an auto-cleared task being swallowed and the LLM repeatedly trying to update task IDs that had vanished from its context.

QA level: light. I've verified the on-disk durability paths manually (corrupt file, deleted directory, missing-ID update) and the included unit tests pass (151 passed, plus biome check and tsc --noEmit clean). I have not end-to-end exercised the auto-clear → system-reminder pathway or the subagent-vanish-during-run pathway in a live session — those rely on session-lifecycle timing I didn't try to synthesize. Treat accordingly; happy to iterate on whatever you'd like tightened, simplified, or reverted.

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Failure modes addressed

• TaskStore.update() returned the same shape for both task missing and task deleted, forcing callers to disambiguate by inspecting fields. The subagent listeners in index.ts ignored the return entirely, so a TaskExecute task that was auto-cleared while the subagent was running would silently swallow the completion event.
• AutoClearManager.onTurnStart returned a boolean, hiding which task IDs were just removed. The LLM had no way to know its earlier-known task IDs had been auto-cleared, so subsequent TaskUpdate calls returned Task #X not found for tasks the user could clearly see in context.
• TaskStore.load() silently swallowed JSON parse errors despite a "start fresh" comment that did not match the code. No signal to the user, no signal to the host.
• mkdirSync ran only in the constructor, so removing the tasks directory mid-stream caused acquireLock and save to throw ENOENT and lose the in-flight mutation.

Changes

• task-store.ts: update() returns notFound: boolean; load() splits ENOENT (silent) from parse errors (calls onCorruptFile callback); ensureDir() runs inside acquireLock and save for auto-heal.
• auto-clear.ts: onTurnStart returns { cleared, ids } so the host can tell the LLM exactly which tasks vanished.
• index.ts: subagent listeners check notFound and surface a UI notice if work completed for a vanished task; tool_result handler drains pendingAutoClearedIds onto the next tool result as a system-reminder so the LLM stops trying to update vanished tasks; pendingAutoClearedIds is reset on session_switch alongside other session-scoped state.
• 6 new tests covering file deletion, dir deletion, missing-id update, successful update return shape, deletion return shape, and corrupt-file preservation.

What I actually verified

Check	Result
biome check src/ test/	clean
tsc --noEmit	clean
vitest run	151 passed (5 files)
Manual: TaskUpdate of nonexistent ID	surfaces "not found" rather than swallowing
Manual: rm -rf .pi/tasks mid-session, then TaskUpdate	dir auto-recreated, mutation persisted
Manual: corrupt tasks-*.json mid-session	in-memory state preserved, onCorruptFile warning toast appeared in pi UI, next save healed the file
Auto-clear pendingAutoClearedIds system-reminder path	not exercised live (logic and unit tests only)
Subagent listener notFound UI notice	not exercised live (logic and unit tests only)

Diffstat: 5 files changed, +248, -30.