feat(cli): add fern docs prettify command

[devin-ai-integration]

Description

Linear ticket: N/A

Adds a new fern docs prettify command that uses AI (via BAML) to analyze markdown documentation files and suggest improvements using Fern's documentation components. The command helps enhance docs by intelligently adding Steps, Cards, Callouts, Code Blocks with tabs, Endpoint snippets, and other Fern components where appropriate.

Changes Made

• Added PrettifyDocs BAML function with comprehensive prompt for analyzing markdown and suggesting Fern component improvements
• Created prettifyDocsCommand.ts handler with:
  • Recursive markdown file discovery (skips node_modules and hidden directories)
  • LLM integration for content analysis
  • Dry-run mode (default: true) to preview changes before applying
  • Detailed change summaries for each file
• Integrated command into CLI as fern docs prettify <path> subcommand under docs
• Added comprehensive Fern components reference embedded in the command for LLM context
• Added version 2.1.0 entry to CLI changelog
• Auto-generated BAML client TypeScript types and functions

Testing

⚠️ This PR has not been manually tested yet. Testing should be done after PR creation:

• Test with single markdown file
• Test with directory containing multiple markdown files
• Test dry-run mode (default behavior)
• Test with --dry-run=false to verify file writing works
• Test with empty files
• Test with files that don't need improvements
• Verify LLM produces reasonable suggestions
• Verify error handling for LLM failures

Human Review Checklist

High Priority:

• Review the BAML prompt in packages/cli/ai/baml_src/main.baml - does it provide clear enough instructions for the LLM?
• Verify file writing logic is safe and won't cause data loss (check prettifyFile function)
• Check that dry-run mode correctly prevents file modifications
• Verify the embedded FERN_COMPONENTS_REFERENCE contains accurate examples of all Fern components

Medium Priority:

• Review file discovery logic - does it correctly skip node_modules and hidden directories?
• Check error handling for LLM API failures
• Verify CLI command registration in cli.ts is correct
• Confirm version bump to 2.1.0 is appropriate for this feature

Low Priority:

• Review auto-generated BAML client code (mostly mechanical changes)
• Consider if the large embedded reference string should be moved to a separate file

Notes

• The command defaults to dry-run mode for safety - users must explicitly pass --dry-run=false to modify files
• Uses the existing BAML DefaultClient which has fallback providers (OpenAI, Anthropic, Bedrock)
• Plant-themed examples are used in the prompt as requested

[devin-ai-integration]

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