feat: add install-skill command

[sonukapoor]

Adds cve-lite install-skill, a 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.

• Single canonical skill content in src/skills/content.ts — one place to update the guidance
• Per-tool adapters write each file to the correct location and format (front-matter for Cursor, invocation hint for Claude Code, ## CVE Lite CLI section for append targets)
• Append-style files (AGENTS.md, GEMINI.md, copilot-instructions.md) are created if missing, appended to if no CVE Lite section exists, or replaced in-place if one already exists — running the command twice is safe
• 233 tests pass

Closes #332