[FEATURE] Add --output-style flag for non-interactive mode

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Currently, output styles work perfectly in interactive mode via /output-style <CUSTOM_STYLE>, but there's no equivalent for non-interactive/programmatic usage with --print mode. This creates a gap for automation workflows that need consistent, structured responses.

Verbose and error-prone

claude --print --append-system-prompt "CRITICAL: Output pure JSON without explanations..."  "Analyze strength & weakness of architecture"

Proposed Solution

# Clean and intuitive
claude --print --output-style architecture-json "Analyze strength & weakness of architecture"

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

CLI commands and flags

Use Case Example

• CI/CD Integration: Automated analysis reports with consistent JSON structure
• Docker/Container Workflows: Microservices orchestration requiring predictable output

formats

• API Integration: Programmatic consumption of Claude responses
• Batch Processing: Large-scale analysis with standardized output schemas

Additional Context

Implementation Details

The feature should:

1. Load output styles from ~/.claude/output-styles/*.md
2. Apply the style's system prompt when --print and --output-style are used together
3. Provide clear error messages for invalid style names

Benefits

• Consistency: Eliminates manual system prompt management
• Maintainability: Centralized style definitions in .md files
• Developer Experience: Matches the intuitive /output-style command from interactive mode
• Automation-Friendly: Essential for programmatic Claude Code usage