Add PR templates to all repos

[zeevdr]

Description

Zero repos have PULL_REQUEST_TEMPLATE.md. PRs frequently merge without updated agent context (per existing /before-pr checklist) or missed docs steps. A template with the pre-PR checklist inline prevents forgetting.

Acceptance criteria

• .github/PULL_REQUEST_TEMPLATE.md in each of the 5 repos
• Sections: Summary, Test plan, Related issues, Checklists (agent context updated, docs regenerated, breaking-change flagged, changelog entry if applicable)
• Shared template in .github org repo where feasible
• Verify template auto-populates on new PRs

[zeevdr]

PRs opened across all repos:

• docs: expand PR template with checklist and related issues #210 — expand template
• docs: expand PR template with checklist and related issues decree-python#26 — expand template
• docs: expand PR template with checklist and related issues decree-typescript#28 — expand template
• docs: add PR template decree-ui#43 — add template (was missing)
• docs: expand PR template with checklist and related issues demos#29 — expand template
• docs: add org-wide PR template fallback .github#16 — org-wide fallback

All include Summary, Related issues, Test plan, and Checklist sections per acceptance criteria. Closed by #210.