feat(cli): add spec command for programmatic access to specifications

Implements new `openspec spec` command with three subcommands:
- `spec show`: Display specs with JSON output and filtering options
- `spec list`: List all available specifications
- `spec validate`: Validate spec structure with detailed reporting

Key features:
- JSON output for CI/CD integration (--json flag)
- Content filtering (--requirements, --no-scenarios, -r)
- Strict validation mode (--strict)
- Support for both Overview/Purpose and Requirements/Behavior sections

This enables programmatic access to OpenSpec specifications for external
tools and automated pipelines as specified in the add-spec-commands change.