refactor: remove schema helper API

[PsiACE]

Summary

• Remove acp.helpers and its top-level re-exports.
• Construct generated schema models directly in examples, contrib code, and golden acceptance cases.
• Update current docs to make direct model construction the standard path.

Related issues

Closes #47

Testing

• make gen-check
• make check
• make test (197 passed, 1 skipped)
• uv run mkdocs build --strict
• Loaded every script under examples/ with the http extra.

No schema regeneration was needed.

Docs & screenshots

README, quickstart, and examples were updated. No visual changes.

Checklist

• Conventional Commit title (e.g. feat:, fix:).
• Tests cover the change or are not required (explain above).
• Docs/examples updated when behaviour is user-facing.
• Schema regenerations (make gen-all) are called out if applicable.