feat(cli): add checks delete command

[martzoukos]

Summary

Adds checkly checks delete <id> (Linear AI-364), following the members delete precedent from #1329:

• New delete(id) method on the Checks REST class (DELETE /v1/checks/:id).
• New checks delete command: destructive/idempotent metadata, required id arg, --force and --dry-run flags, and the standard confirmOrAbort preview (interactive confirmation, agent-mode exit 2 without --force).
• The check is fetched first so the confirmation names what's being deleted; a 404 produces a friendly Check "<id>" not found. It may have already been deleted. message and exit 1.
• Since the checks API exposes no project-membership field, both the help text and the confirmation preview carry an unconditional warning that checks managed by a CLI project are recreated on the next checkly deploy and should be removed from project code instead.

Test plan

• Unit tests in checks-delete.spec.ts: exit 2 without --force in agent mode, execution with --force, --dry-run preview, not-found handling, delete failure exit code, metadata.
• Full suite green (1203 passed, 88 files), lint clean, build succeeds, skills/checkly/SKILL.md in sync.
• Manual smoke test against a real account: --dry-run resolves the check and prints the preview envelope.

🤖 Generated with Claude Code

[martzoukos]

Faced this shortcoming in my prompt-onboarding agent flow, so figured it might be good to be added.