feat: cve-lite install-skill command

[sonukapoor]

Summary

Add a cve-lite install-skill subcommand that writes AI assistant skill files into the current project directory for five tools: Claude Code, Codex CLI, Gemini CLI, Cursor, and GitHub Copilot.

Running the command once benefits the whole team — the files are committed to the repo and every developer who clones it gets the skills automatically.

Behaviour

• Writes skill files to tool-specific locations (.claude/commands/cve-lite.md, AGENTS.md, GEMINI.md, .cursor/rules/cve-lite.mdc, .github/copilot-instructions.md)
• For files that may already have other content (AGENTS.md, GEMINI.md, copilot-instructions.md), the CVE Lite section is appended or replaced on re-run — existing content is preserved
• Running install-skill twice is safe (idempotent)

Acceptance criteria

• cve-lite install-skill creates all five skill files in the current working directory
• Re-running does not duplicate content
• Existing file content outside the CVE Lite section is preserved
• Command exits 0 on success, 1 on file write error