[BUG] v2.1.50 Regression: Completed tasks nuked from disk — memory leak fix deletes task files instead of freeing references

Bug Description

In v2.1.50, marking a task as completed via TaskUpdate causes the task JSON file to be immediately removed from ~/.claude/tasks/<session-id>/. Calling TaskList afterward shows no completed tasks. They are gone, not just hidden.

This is a regression. In v2.1.49 and earlier, completed tasks remained visible in TaskList with [completed] status and their JSON files persisted on disk.

Steps to Reproduce

1. Start a fresh session on v2.1.50 (not a resumed session)
2. Create a task: TaskCreate { subject: "Test task", description: "Testing" }
3. Mark it completed: TaskUpdate { taskId: "1", status: "completed" }
4. Call TaskList — task is gone
5. Check disk: ls ~/.claude/tasks/<session-id>/ — only .lock and .highwatermark, no task JSON files

Expected Behavior

Completed tasks remain in TaskList with status completed and persist on disk as JSON files in ~/.claude/tasks/<session-id>/.

Actual Behavior

Completed task JSON files are deleted from disk immediately upon completion. TaskList returns no record of them.

Key Observations

• Resumed sessions from pre-v2.1.50 are NOT affected — their completed tasks persist normally. Only fresh v2.1.50 sessions exhibit this behavior.
• The .highwatermark file appears only in v2.1.50 task directories and may be related.
• Tested across multiple agents and projects — all fresh v2.1.50 sessions are affected.
• status: "completed" is confirmed via transcript analysis — agents are NOT calling status: "deleted".

Likely Cause

v2.1.50 includes three task-related memory leak fixes:

• "Fixed memory leak where completed task state objects were never removed from AppState"
• "Fixed memory leak where completed task output was not freed from memory"
• "Fixed memory leak where TaskOutput retained recent lines after cleanup"

The first fix appears to be removing completed task files from disk rather than only freeing in-memory references.

Environment

• Platform: Linux (Ubuntu, x86_64)
• Version: 2.1.50
• Model: Claude Opus 4.6