docs: add PR template + 8 beginner-friendly issues

.github/pull_request_template.md

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+## Summary
+
+<!-- What does this PR do? 1-3 bullet points. -->
+
+-
+
+## Why
+
+<!-- What problem does it solve? Link related issues with "Closes #123". -->
+
+## Changes
+
+<!-- List key changes. For new skills/presets, describe what they cover. -->
+
+-
+
+## Test Plan
+
+- [ ] Existing tests pass (`pytest --ignore=agent/tests/e2e_backtest --tb=short -q`)
+- [ ] New tests added (if applicable)
+- [ ] Tested manually (describe below)
+
+## Checklist
+
+- [ ] No changes to protected areas (`src/agent/`, `src/session/`, `src/providers/`) without prior discussion
+- [ ] No hardcoded values (API keys, file paths, magic numbers)
+- [ ] Code follows [CONTRIBUTING.md](../CONTRIBUTING.md) guidelines
+- [ ] Documentation updated (if user-facing change)