Add shared PR Definition of Done template

[EmersonBraun]

What

Add a concise organization-default PR template with What, Why, How, Reuse, and Validation sections plus the common Definition of Done checklist.

Why

Repositories without their own PR template inherit this template, giving contributors a shared, actionable bar for acceptance tests, documented checks, generated artifacts, and release/documentation updates.

How

The checklist covers one package or isolated cross-cutting gate per PR; contract or edge-case tests per acceptance criterion; documented gates on the current commit; conditional size measurement and JSDoc; required changeset/docs; no disabled tests; and official generated-artifact regeneration. The accompanying report records minimal alignment diffs for agentskit, harness, and code-review without editing those repositories: /Users/rebecabraun/Documents/ChatGPT/New project/audit-evidence/v2/q04-dod/report.md.

Validation

• git diff --cached --check passed before commit.
• Confirmed branch codex/pr-template-dod is based on origin/main; commit 13791d6 is pushed.
• Read the three existing templates and audit control sources used for the checklist and alignment report.
• No automated tests were run for this Markdown-only change.
• No merge performed.