feat: add hive config validate command

[hay-kot]

Summary

• Add hive config validate command to test configuration files before execution
• Validates file access, template syntax, regex patterns, and keybindings
• Supports --format json for tooling integration
• Exits with code 1 if validation fails

Validation Checks

1. File Access - Config file readable, git executable found, data directory accessible
2. Template Syntax - Spawn commands and keybinding shell commands parse correctly
3. Pattern Syntax - Hook patterns are valid regex (warns if patterns look like globs)
4. Keybindings - Valid actions, no conflicts between action/sh fields

Example Output

$ hive config validate
✔ File Access: File paths validated
  Config file: ~/.config/hive/config.yaml (found)
  Git path: /opt/homebrew/bin/git (found)
  Data directory: ~/.local/share/hive (exists)
✔ Spawn Commands: 1 command(s) defined
  Command 0: valid template
✔ Recycle Commands: 3 command(s) defined
✔ Hooks: 3 hook(s) defined
✔ Keybindings: 4 keybinding(s) defined

✔ Configuration is valid

Test plan

• Unit tests for all validation scenarios
• Manual testing with valid config
• Manual testing with invalid templates, regex, and keybindings
• JSON output format verified
• Exit code 1 on validation failure

Closes #3