Use chroma style registry

[toshimaru]

Summary

• replace local style definitions with chroma's built-in styles package
• rely on chroma's embedded XML styles for theme listing

Related PR

• Use own syntax-highlight theme #6

Testing

• go build -o nyan
• go test ./...

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https://chatgpt.com/codex/tasks/task_e_68b8e55bc300832b8fe349c8a8e528a3

[Copilot]

Pull Request Overview

This PR replaces local style definitions with Chroma's built-in styles package, eliminating custom style implementations in favor of the upstream library's embedded styles.

• Removes all custom style definition files (11 style files)
• Updates import to use Chroma's styles package instead of local implementation
• Removes local style registry and API functionality

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

File	Description
styles/*.go	Deletes all custom style definitions (abap, dracula, emacs, monokai, etc.)
styles/api.go	Removes local style registry and management functions
styles/api_test.go	Removes tests for local style API
cmd/root.go	Updates import to use chroma's styles package

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[toshimaru]

Related PR #6