Convert issue templates to YAML Issue Forms with SPDX header

.github/ISSUE_TEMPLATE/bug_report.yml

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+# Copyright (c) The mlkem-native project authors
+# SPDX-License-Identifier: Apache-2.0 OR ISC OR MIT
+
+name: Bug report
+description: Create a report to help us improve
+labels: [bug]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        **Security reports**
+
+        DO NOT report security issues through GitHub issues - instead use GitHub's [private vulnerability reporting](https://github.com/pq-code-package/mlkem-native/security).
+  - type: textarea
+    id: describe
+    attributes:
+      label: Describe the bug
+      description: A clear and concise description of what the bug is.
+    validations:
+      required: true
+  - type: input
+    id: platform
+    attributes:
+      label: Platform
+      placeholder: e.g., x86_64 Linux
+    validations:
+      required: true
+  - type: input
+    id: compiler
+    attributes:
+      label: Compiler
+      placeholder: e.g., gcc 13.2.0 (installed through nix)
+    validations:
+      required: true
+  - type: input
+    id: version
+    attributes:
+      label: mlkem-native version
+      placeholder: e.g., v1.0.0 or main-branch
+    validations:
+      required: true
+  - type: textarea
+    id: reproduce
+    attributes:
+      label: How to reproduce
+      description: Steps to reproduce the behavior.
+      render: shell
+    validations:
+      required: true
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected behavior
+      description: A clear and concise description of what you expected to happen.
+    validations:
+      required: true
+  - type: textarea
+    id: context
+    attributes:
+      label: Additional context
+      description: Add any other context about the problem here.

.github/ISSUE_TEMPLATE/feature_request.yml

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+# Copyright (c) The mlkem-native project authors
+# SPDX-License-Identifier: Apache-2.0 OR ISC OR MIT
+
+name: Feature request
+description: Suggest an idea for this project
+labels: [enhancement]
+body:
+  - type: checkboxes
+    id: platform
+    attributes:
+      label: Platform
+      description: Which platform does this concern?
+      options:
+        - label: platform independent
+        - label: aarch64
+        - label: x86_64
+        - label: rv64
+        - label: other
+  - type: checkboxes
+    id: category
+    attributes:
+      label: Issue category
+      description: Which part(s) of mlkem-native does this issue concern?
+      options:
+        - label: "core crypto: ML-KEM"
+        - label: "core crypto: FIPS202"
+        - label: documentation
+        - label: integration
+        - label: CBMC
+        - label: CI
+        - label: Testing
+  - type: textarea
+    id: solution
+    attributes:
+      label: Describe the solution you'd like
+      description: A clear and concise description of what you want to happen.
+    validations:
+      required: true
+  - type: textarea
+    id: context
+    attributes:
+      label: Additional context
+      description: Add any other context or screenshots about the feature request here.