add testing principles

[atkrad]

Summary

• Add a testing principles page for how Deployah tests are named, organized, and reviewed.
• Point the contributor guide at that page instead of repeating the rules.

Test plan

• nix run .#lint-md
• Markdown link check (lychee --offline --include-fragments)
• Unit tests added/updated (docs only; no behavior change)
• Scenario under scenarios/ (no deploy behavior change)
• Pre-commit hooks passed on the commit (markdownlint, markdown links)

Labels

• One of: kind/feature, kind/bug, kind/docs, kind/chore
• Add breaking-change if this breaks existing CLI or config behavior
• Add skip-changelog for internal-only PRs that should not appear in notes

Checklist

• Title is short and imperative (matches commit style)
• Docs / CLI help updated when user-facing (README.md, docs/cli/)
• No secrets or local-only paths in the diff

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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