ci(pr-template): Add PR template for planning documents

.github/PULL_REQUEST_TEMPLATE/planning.md

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+## Summary
+
+<!-- Brief description of the planning document(s) being added or modified -->
+
+## Planning Document Type
+
+- [ ] Plan (`.ai/plans/<plan-name>.md`)
+- [ ] Phase document (`.ai/plans/phases/<plan-name>-phase-<N>.md`)
+- [ ] ADR - Architecture Decision Record (`.ai/docs/adr/*.md`)
+- [ ] Index update (`.ai/INDEX.md`)
+- [ ] Other planning artifact: <!-- specify -->
+
+## Documents Affected
+
+| File | Action | UUID |
+|------|--------|------|
+| `.ai/plans/example.md` | Added/Modified | `XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX` |
+
+## Related Issues
+
+<!-- Link to related issues: Fixes #123, Relates to #456 -->
+
+## Frontmatter Checklist
+
+### Required Fields
+- [ ] `id` - Valid UUID v4 (uppercase with hyphens)
+- [ ] `title` - Descriptive document title
+- [ ] `status` - Emoji prefix + status text (e.g., `⏳ In Progress`, `✅ Completed`)
+- [ ] `date` - ISO 8601 format (YYYY-MM-DD)
+- [ ] `author` - Author username
+
+### Optional Fields (if applicable)
+- [ ] `related` - Array of related document UUIDs
+- [ ] `children` - Array of child document UUIDs (for plans with phases)
+- [ ] `depends_on` - Array of dependency UUIDs (for phase documents)
+- [ ] `tags` - Array of categorization tags
+
+## Plan Structure (for new plans)
+
+- [ ] Problem statement clearly defined
+- [ ] Goals/objectives listed
+- [ ] Phases broken down (if multi-phase)
+- [ ] Success criteria defined
+- [ ] Dependencies identified
+
+## Cross-Reference Verification
+
+- [ ] Bidirectional links maintained (if A `related` to B, B `related` to A)
+- [ ] Parent plan references child phases in `children` array
+- [ ] Child phases reference parent in `related` array
+- [ ] INDEX.md updated to include new documents
+
+## Phase Document Checklist (if applicable)
+
+- [ ] Phase numbered correctly in filename
+- [ ] Tasks/steps clearly defined
+- [ ] Acceptance criteria specified
+- [ ] Dependencies on prior phases documented
+
+## ADR Checklist (if applicable)
+
+- [ ] Problem statement clearly articulated
+- [ ] Decision documented
+- [ ] Alternatives considered and documented
+- [ ] Tradeoffs analyzed
+- [ ] Implementation strategy outlined
+
+## Validation
+
+- [ ] All UUIDs are unique (not duplicated from other documents)
+- [ ] All referenced UUIDs exist in the project
+- [ ] Markdown renders correctly
+- [ ] No broken internal links
+
+## Additional Context
+
+<!-- Any other information reviewers should know about these planning documents -->