feat: add local format conversion command

[pchuri]

Description

Add a convert command for offline conversion between content formats (markdown, storage, html, text) without requiring a Confluence server connection.

Resolves #91

Supported conversions:

From	To
markdown	storage, html, text
html	storage, markdown, text
storage	markdown, html, text

Usage:

# stdin/stdout
echo "# Hello" | confluence convert --input-format markdown --output-format storage

# file-based
confluence convert -i doc.md -o doc.xml --input-format markdown --output-format storage

Type of Change

• New feature (non-breaking change which adds functionality)

Testing

• Tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

Implementation Details

• Added ConfluenceClient.createLocalConverter() static factory that creates a lightweight instance with only the markdown parser, skipping HTTP client setup
• Supports --input-file/--output-file options (defaults to stdin/stdout for piping)
• 12 new tests covering all conversion paths and error cases
• Updated README.md and SKILL.md documentation

[github-actions]

🎉 This PR is included in version 1.28.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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