Add CONTRIBUTING.md and GitHub issue/PR templates for incoming changes

[xubinrui]

Problem

The repository now has active inbound work, with open pull requests already proposing product and scaffold changes. At that point, contributor expectations should move out of ad hoc PR descriptions and into repository-level guidance.

Right now there is no CONTRIBUTING.md, no issue template, and no pull request template in the repository. That makes it harder to keep submissions consistent, communicate the expected scope of changes, and collect the minimum context maintainers need to review incoming work efficiently.

Why this is distinct from current open work

• Issue README lacks basic project context and onboarding information #1 is about improving repository-facing documentation in README.md.
• PRs Add a polished static OpenAI-compatible chat client #2, Add minimal repository scaffold docs and ignore rules #3, and Add repository scaffold README and gitignore #4 are focused on application/scaffold content changes.
• This issue is specifically about repository maintenance process: contributor workflow, submission expectations, and review hygiene.

Proposal

Add:

• a top-level CONTRIBUTING.md that explains contribution expectations, setup/validation expectations, branch naming or commit guidance if desired, and what a good issue or PR should include
• one or more issue templates under .github/ISSUE_TEMPLATE/ for bug reports and feature/change requests
• a .github/pull_request_template.md that asks contributors for summary, motivation, testing, and related issue links

Suggested acceptance criteria

• CONTRIBUTING.md exists and is linked from the README or otherwise discoverable
• issue templates guide reporters toward actionable reproduction steps and expected context
• the PR template asks for change summary, verification performed, and related issues
• the new documents are lightweight and match the repository's current size instead of over-engineering the process

Why now

A small amount of workflow structure will reduce review churn immediately, especially while the repository is still forming its baseline conventions and multiple independent changes are already arriving.