refactor: convert IDE installers to flat structure with shared methods

[alexeyv]

Summary

• Extracts shared flat-file install methods to base class for code reuse
• Converts claude-code, cline, and antigravity installers to flat structure
• Converts cursor, windsurf, crush, and github-copilot installers to flat structure
• Updates documentation for all converted installers

Test plan

• Run npm run test:install to verify installer functionality
• Test installation to a sample project for each converted IDE

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