chore(workflows): align with internal template

[shadowhand]

Summary by cubic

Aligns the repository with the internal project template, updating tooling config, CI, and development guidelines without changing runtime behavior or the public API.

Tooling and CI

• Updates composer scripts, PHPUnit config, Mago config, .editorconfig, .gitignore, and the test workflow to match the template.
• Replaces tmp/ with build/ for coverage and cache artifacts.
• Removes workflow_call from the test workflow and adds contents: read permissions.
• Drops the ergebnis/composer-normalize dev dependency.

Code style and docs

• Rewrites AGENTS.md with new project guidelines, including no longer preserving backward compatibility.
• Adds use function and use const imports across src/ and tests/ to satisfy new Mago lint rules.
• Replaces fully-qualified exception and class references (e.g., \InvalidArgumentException, \stdClass) with imports.

^{Written for commit 7e4d6bb. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 66 files (changes from recent commits).

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