[Task]: Standardize issue forms, PR template, and contribution workflow #67

Motivation

The repo has contribution guidance but no issue forms or pull request template,
and the guide predates the current just commands and CI workflow. Contributors
need one clear process that keeps changes scoped, reviewable, tested, and linked
to tracked work.

Proposed change

• Add issue forms for bugs, features, and engineering tasks, plus chooser config
  that disables blank issues and points open-ended ideas to Discussions.
• Add a pull request template with summary, related-issue keywords, testing, and
  an author checklist.
• Rewrite CONTRIBUTING.md around the current just-based workflow.

Affected modules or data

• New: .github/ISSUE_TEMPLATE/*, .github/pull_request_template.md
• Rewrite: CONTRIBUTING.md
• No app code or CI changes.

Acceptance criteria

• Issue forms and chooser config render and behave as intended.
• PR template and CONTRIBUTING.md document the required workflow (one issue
  per PR, squash merge by default, linked issues, test and data-validation
  requirements, AI-assisted ownership).
• Existing CI stays green; no application behavior changes.

Testing requirements

• Validate issue form YAML against GitHub's schema and run prettier.
• No code changes, so just check and CI behavior are unaffected.

Dependencies and risks

• None. Issue forms and the PR template only render in the GitHub UI after this
  merges to main.