chore: add GitHub issue and PR templates

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: Bug Report
+description: Report a bug or unexpected behavior
+labels: ["bug"]
+body:
+  - type: textarea
+    id: description
+    attributes:
+      label: Describe the bug
+      description: A clear description of what the bug is.
+    validations:
+      required: true
+  - type: textarea
+    id: steps
+    attributes:
+      label: Steps to reproduce
+      description: How can we reproduce this?
+      placeholder: |
+        1. Open Anvil
+        2. Create a new thread
+        3. ...
+    validations:
+      required: true
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected behavior
+      description: What did you expect to happen?
+    validations:
+      required: true
+  - type: textarea
+    id: actual
+    attributes:
+      label: Actual behavior
+      description: What actually happened?
+    validations:
+      required: true
+  - type: dropdown
+    id: os
+    attributes:
+      label: Operating System
+      options:
+        - macOS
+        - Windows
+        - Linux
+    validations:
+      required: true
+  - type: input
+    id: version
+    attributes:
+      label: Anvil Version
+      placeholder: "e.g., v0.0.8"
+    validations:
+      required: true
+  - type: textarea
+    id: logs
+    attributes:
+      label: Relevant logs
+      description: Paste any relevant logs from `logs/dev.log` or the developer console.
+      render: shell

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Discord Community
+    url: https://discord.gg/tbkAetedSd
+    about: Ask questions, get help, and discuss Anvil with the community.
+  - name: Anvil Website
+    url: https://www.anvil.fm
+    about: Download Anvil and learn more about the project.

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: Feature Request
+description: Suggest a new feature or improvement
+labels: ["enhancement"]
+body:
+  - type: textarea
+    id: problem
+    attributes:
+      label: Problem
+      description: What problem does this solve? What workflow is painful today?
+    validations:
+      required: true
+  - type: textarea
+    id: solution
+    attributes:
+      label: Proposed solution
+      description: How would you like this to work?
+    validations:
+      required: true
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives considered
+      description: What other approaches have you considered?
+  - type: textarea
+    id: context
+    attributes:
+      label: Additional context
+      description: Screenshots, mockups, or any other relevant information.

.github/PULL_REQUEST_TEMPLATE.md

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+## Summary
+
+<!-- What does this PR do? Why? -->
+
+## Changes
+
+<!-- Bullet list of key changes -->
+
+## Test Plan
+
+<!-- How was this tested? -->
+
+- [ ] `pnpm test` passes
+- [ ] `pnpm lint` passes
+- [ ] Manual testing performed (describe below)
+
+## Screenshots
+
+<!-- If applicable, add screenshots or screen recordings -->