Add GitHub issue and PR templates

[crtahlin]

Sub-issue of #114 (developer experience review).

Task

Create:

• .github/ISSUE_TEMPLATE/bug_report.md — description, steps to reproduce, expected/actual, environment (Python, OS, backend, version), logs
• .github/ISSUE_TEMPLATE/feature_request.md — problem/motivation, proposed solution, alternatives, context
• .github/pull_request_template.md — summary, related issues, change type, checklist (tests pass, lint passes, version bumped, changelog updated, docs updated)

Why

No templates exist. Standardized templates help both agents and humans submit consistent, complete issues and PRs.

[miles-on-nightshift]

Triage: classification: enhancement, priority: priority-low

Issue/PR templates improve contribution quality but are not blocking.

[miles-on-nightshift]

Triage: docs / priority: low

Issue/PR templates help with community contributions once public. Low urgency.

Labels to add: documentation, priority-low