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/workflows/file-structure-standards.md

@@ -105,7 +105,7 @@ impl-2              # Another main task
 
 ### Directory Permissions and Access
 - All workflow directories are project-local
-- Session registry at `.workflow/session_status.jsonl` (global)
+- Active session marked by `.workflow/.active-[session-name]` marker file (global)
 - Individual sessions in `.workflow/WFS-[topic-slug]/` (session-specific)
 
 ## Document Generation Triggers