feat(cli): add clean command to prune sessions and checkpoints

[HarshRajSinghania]

Summary

Adds a terminal-agent clean command to prune old session, checkpoint, and telemetry artifacts under .terminal_agent/.

Motivation

Implements #2. Session and checkpoint data can accumulate during development and benchmark runs with no CLI way to reclaim disk space.

Implementation

• New command: terminal-agent clean [--all] [--days N] [--dry-run] [-C DIR]
• --days defaults to 7; --all ignores age and removes all matching artifacts
• --dry-run lists file count and size (MB) without deleting
• Empty or missing .terminal_agent exits 0 with an informative message
• Parent directories (sessions/, checkpoints/) are kept
• Helpers: SessionManager.delete_sessions_older_than, CheckpointManager.prune_checkpoints
• Command registered in cli/main.py and KNOWN_COMMANDS

Testing

PYTHONPATH=src python3 -m pytest tests/unit/test_clean_command.py -q

Result: 7 passed

Also ran the existing tests/unit/test_config.py collection locally after installing typer, rich, pydantic, gitpython, pyyaml, psutil from PyPI.

Closes #2

[Kaap10]

Thanks for the contribution, @HarshRajSinghania.
The implementation is clean and all test checks have passed. Merging now.