feat(cli): add the skill deletion command, and simultaneously improve the help information.

[breath57]

Description

This PR adds the skills delete command and improves the help information for the skills management CLI.

Changes

1. New skills delete command

Added a new subcommand to delete skills:

# Delete a user-level skill
deepagents skills delete my-skill

# Delete skill for a specific agent
deepagents skills delete my-skill --agent mybot

# Delete a project-level skill
deepagents skills delete my-skill --project

# Force delete (skip confirmation)
deepagents skills delete my-skill --force

Features:

• Shows file count before deletion
• Interactive confirmation prompt (can be skipped with --force)
• Supports --agent and --project options

2. Improved help information

Enhanced the deepagents skills help output to include:

• All available commands (including the new delete command)
• Common options section explaining --agent and --project
• More comprehensive examples
• Skill locations section showing storage paths

Before:

Available commands:
  list              List all available skills
  create <name>     Create a new skill
  info <name>       Show detailed information about a skill

After:

Available commands:
  list              List all available skills
  create <name>     Create a new skill
  info <name>       Show detailed information about a skill
  delete <name>     Delete a skill

Common options:
  --agent <name>    Specify agent identifier (default: agent)
  --project         Use project-level skills instead of user-level

Skill locations:
  User skills:      ~/.deepagents/<agent>/skills/
  Project skills:   <project>/.deepagents/skills/

Testing

Added 11 new test cases in tests/unit_tests/skills/test_commands.py:

• test_delete_existing_skill_with_force
• test_delete_nonexistent_skill
• test_delete_with_confirmation_yes
• test_delete_with_confirmation_no
• test_delete_with_confirmation_empty_input
• test_delete_with_keyboard_interrupt
• test_delete_invalid_skill_name
• test_delete_project_skill
• test_delete_project_skill_not_in_project
• test_delete_skill_with_supporting_files
• test_delete_skill_for_specific_agent

All tests pass

Checklist

• Unit tests added
• Code follows existing patterns
• Security validations included (name validation, path traversal prevention)
• Backwards compatible (no breaking changes)