meta: add PR template and branching hygiene guidance

.github/pull_request_template.md

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+## Task / context
+- Mission Control task: <link or id>
+- Why: <what problem this PR solves>
+
+## Scope
+- <bullet 1>
+- <bullet 2>
+
+### Out of scope
+- <explicitly list what is NOT included>
+
+## Evidence / validation
+- [ ] `make check` (or explain what you ran instead)
+- [ ] E2E (if applicable): <cypress run / screenshots>
+- Logs/links:
+  - <link to CI run>
+
+## Screenshots (UI changes)
+| Desktop | Mobile |
+| --- | --- |
+| <img src="..." width="600" /> | <img src="..." width="300" /> |
+
+## Docs impact
+- [ ] No user/operator docs changes required
+- [ ] Docs updated: <paths/links>
+
+## Risk / rollout notes
+- Risk level: low / medium / high
+- Rollback plan (if needed): <steps>
+
+## Checklist
+- [ ] Branch created from `origin/master` (no unrelated commits)
+- [ ] PR is focused (one theme)
+- [ ] No secrets in code/logs/docs
+- [ ] If API/behavior changes: docs updated (OpenAPI + `docs/reference/api.md`)