Align pull request template with AI policy disclosure

Update the pull request template to match the AI disclosure standards
introduced in the issue templates:

- Use relative link to AI_POLICY.md instead of absolute GitHub URL
- Update dropdown wording to match issue forms
- Use open-weight models in placeholder examples
- Use FOSS tools in placeholder examples
- Add 'How was AI used?' section for the partial-usage explanation

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,33 @@
+<!--
+  Before submitting this pull request, please read the AI Policy:
+  https://github.com/iv-org/invidious/blob/master/AI_POLICY.md
+
+  If you used AI (fully or partially) to produce any content in this PR,
+  you MUST disclose it per the policy, including the exact model(s) and tool(s) used.
+-->
+
+## Checklist
+
+- [ ] I have read the [AI Policy](./AI_POLICY.md) and understand the disclosure requirements
+
+## AI Usage
+
+<!-- Tick exactly one -->
+
+- [ ] AI was not used to write this pull request
+- [ ] AI was used to fully write this pull request
+- [ ] AI was used to partially create this pull request (explain how)
+
+## AI Disclosure
+
+<!-- Leave blank if not applicable -->
+
+**Model(s) used:**
+<!-- e.g. Llama 3.1, Mistral, Qwen 2.5, DeepSeek-R1 -->
+
+**Tool(s) used:**
+<!-- e.g. Ollama, Open WebUI, Continue -->
+
+## How was AI used?
+
+<!-- If you selected "AI was used to partially create this pull request (explain how)" above, explain how AI was used here. Leave empty otherwise. -->