chore: add issue and PR templates

Bug report and feature request templates only — no blank issues,
no support requests. PR template with test checklist.

Author: bakerboy448

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: Bug Report
+description: Report a bug in the sanitizer
+labels: [bug]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        **This tracker is for bugs only.** For support questions, visit the relevant community channels (e.g., Discord, forums).
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+      description: What happened?
+    validations:
+      required: true
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected behavior
+      description: What should have happened?
+    validations:
+      required: true
+  - type: textarea
+    id: input
+    attributes:
+      label: Input YAML (sanitized)
+      description: Paste a minimal example that reproduces the issue. Redact any real secrets first.
+      render: yaml
+  - type: textarea
+    id: output
+    attributes:
+      label: Actual output
+      description: What the sanitizer produced
+      render: yaml
+  - type: input
+    id: browser
+    attributes:
+      label: Browser
+      description: e.g., Chrome 120, Firefox 121, Safari 17

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links: []

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: Feature Request
+description: Suggest an improvement or new feature
+labels: [enhancement]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        **This tracker is for feature requests only.** For support questions, visit the relevant community channels.
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+      description: What would you like to see added or changed?
+    validations:
+      required: true
+  - type: textarea
+    id: use-case
+    attributes:
+      label: Use case
+      description: Why is this needed? What problem does it solve?
+    validations:
+      required: true
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives considered
+      description: Have you considered any workarounds?

.github/pull_request_template.md

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+## Summary
+
+<!-- Brief description of changes -->
+
+## Changes
+
+<!-- Bullet list of what changed -->
+
+## Test plan
+
+<!-- How to verify this works -->
+
+- [ ] Tests pass (`npm test`)
+- [ ] TypeScript compiles (`npx tsc --noEmit`)
+- [ ] Build succeeds (`npm run build`)