AD-1002: Add Contributor Workflow

[alph-cmky]

Source: docs/development/task-backlog.md

Milestone: Phase 10: Packaging and Examples

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AD-1002: Add Contributor Workflow

Goal: Make open-source contribution flow explicit.

Files:

• Modify docs/contributing.md
• Create .github/pull_request_template.md
• Create .github/workflows/ci.yml

Tasks:

• Document branch naming, commit style, and local verification commands.
• Add PR checklist for tests, docs, and security-sensitive behavior.
• Add CI steps for install, lint, typecheck, and test.
• Commit with message chore: add contributor workflow.

Acceptance Criteria:

• Pull requests have the same quality gates as local development.