docs: clarify validation and PR workflow

[rlippmann]

Summary

Clarify repository validation expectations and clean up the pull request workflow documentation.

Changes in this PR:

• Clarify validation expectations in AGENTS.md
• Clarify validation expectations in CONTRIBUTING.md
• Explicitly map changed file types to the relevant validation commands
• Require reporting validation commands and results before reporting completion
• Clarify behavior when a required validation step cannot be run
• Remove the redundant # Pull Request heading from the PR template
• Keep Python pre-commit optional and avoid requiring Python tooling for TypeScript contributors

Why

Recent work exposed a recurring failure mode where focused tests were run successfully but CI later failed on formatting or documentation validation.

The repository already had validation guidance, but it was not explicit about which validation paths should be run when Python, TypeScript, Markdown, or multiple file types are modified.

This PR makes those expectations clearer for both human contributors and coding agents.

The PR template cleanup removes a redundant top-level heading that was appearing in rendered pull request descriptions.

Example metadata

• Enforcement point: N/A
• Domain: Repository governance
• Technology/runtime: Documentation
• Example type: repository maintenance
• Does this use directive drafter? should normally be no: No
• Does this derive state from model output? must be no for primary examples: No

Checklist

• Repository scaffolding / governance change
• One primary enforcement point
• Explicit authoritative state
• No model-derived state mutation
• Adversarial stub or equivalent test
• Observable runtime behavior change
• Domain vocabulary is natural
• Framework is secondary to the enforcement point
• Docs updated
• Tests/smoke checks added where practical

Notes

Validation:

npx --yes markdownlint-cli2 AGENTS.md CONTRIBUTING.md .github/pull_request_template.md