refactor(core): Boris Cherny cleanup - type consolidation and dependency fix

## Sprint 1: ExecutionResult Consolidation (7 → 1)
- Kept canonical definition in jdev_cli/core/execution.py
- Renamed duplicates to distinct types:
  - ToolExecutionResult (shell_bridge.py)
  - StreamingExecutionResult (streaming/executor.py)
  - ParallelExecutionResult (async_executor.py)
  - SandboxResult (python_sandbox.py)
  - SafeExecutionResult (safe_executor.py)
- sandbox.py now imports from canonical source

## Sprint 2: Fixed Broken References
- Updated docstrings from third_party → jdev_governance
- Updated docstrings from qwen_dev_cli → jdev_cli
- Fixed codebase_path in shell_main.py

## Sprint 3: Backup Cleanup
- Removed 16 backup files and directories
- Cleaned .qwen_backups, .qwen_atomic_backups, .streaming_backup
- Removed reviewer.py.backup_*, base.py.backup_*

## Sprint 4: Circular Dependency Break
- Created jdev_cli/ui_launcher.py as lazy loader
- jdev_cli no longer directly imports jdev_tui
- Dependency direction: jdev_tui → jdev_cli (correct)

## Sprint 5: Plugin System Fix
- Added jdev_cli/plugins/__init__.py with Plugin protocol
- Plugin system now properly importable

## Metrics After Cleanup
- ExecutionResult classes: 7 → 1 (canonical)
- Backup files: 16 → 0
- Circular dependencies: 5 → 0
- Missing __init__.py: 2 → 0

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

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

Author: JuanCS-Dev