feat(cli): add skills install command

[stefanjudis]

Summary

• Adds npx checkly skills install command to install the Checkly agent skill (SKILL.md) into a project
• Supports 12 platforms via --target (Claude, Cursor, Windsurf, GitHub Copilot, Goose, Continue, AMP, Cline, Codex, Gemini CLI, OpenCode, Roo)
• Supports --path for custom directories and --force to skip overwrite confirmation
• Interactive prompt when run in a TTY without flags; prints usage guidance in non-interactive mode (CI, piped)
• Moves skills.ts → skills/index.ts to support subcommands under the skills topic

New files

• packages/cli/src/commands/skills/install.ts — The command
• packages/cli/src/commands/skills/__tests__/install.spec.ts — Unit tests (22 tests)

Modified files

• packages/cli/src/commands/skills/index.ts — Moved from skills.ts, import paths updated
• packages/cli/package.json — Remove explicit skills topic description (now handled by the index command)
• packages/cli/src/commands/__tests__/command-metadata.spec.ts — Register skills install
• packages/cli/src/help/help-extension.ts — Exclude topics that have a matching top-level command from duplicating in help output
• packages/cli/src/ai-context/README.md — Document install command
• packages/cli/src/ai-context/skill.md — Document install command
• skills/checkly/README.md — Document install command
• skills/checkly/SKILL.md — Document install command

Test plan

• All 22 unit tests pass (vitest run)
• Command metadata tests pass (including new skills install)
• ESLint passes via pre-commit hook
• Manual test: npx checkly skills install (interactive prompt)
• Manual test: npx checkly skills install --target claude
• Manual test: npx checkly skills install --target claude --force
• Manual test: npx checkly skills install --path custom/dir
• Manual test: non-interactive mode
• Manual test: overwrite confirmation when SKILL.md already exists

🤖 Generated with Claude Code and reviewed / manually tested by @stefanjudis