Replace JSONL session registry with marker file system

- Replace complex .workflow/session_status.jsonl registry with simple .active-[name] marker files
- Implement ultra-fast session detection using file existence checks (<1ms vs JSON parsing)
- Add atomic session switching through file creation/deletion operations
- Remove all session_status.jsonl references across 9 command files
- Rewrite session-management-principles.md with new marker file architecture
- Update file-structure-standards.md directory access documentation
- Maintain backward compatibility with automatic migration from JSONL to marker files
- Enable visual session management through standard filesystem commands (ls, touch, rm)
- Eliminate JSON parsing overhead and corruption risks in session management

Core benefits: Zero parsing overhead, atomic operations, visual management, self-healing consistency

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.claude/commands/task/breakdown.md

@@ -23,7 +23,7 @@ Intelligently breaks down complex tasks into manageable subtasks with automatic
 ⚠️ **CRITICAL**: Before breakdown, MUST check for existing active session to avoid creating duplicate sessions.
 
 ### Session Check Process
-1. **Query Session Registry**: Check `.workflow/session_status.jsonl` for active sessions. If the file doesn't exist, create it.
+1. **Check Active Session**: Check for `.workflow/.active-*` marker file to identify active session containing the parent task.
 2. **Session Validation**: Use existing active session containing the parent task
 3. **Context Integration**: Load existing session state and task hierarchy