chore: move .github folder to .gitea for Gitea compatibility

Gitea prefers .gitea/ISSUE_TEMPLATE/ and .gitea/workflows/ over the
GitHub-convention .github/ equivalents. Moves all issue templates and
workflow files to the Gitea-native paths and updates CLAUDE.md references.

Cosign certificate identity paths in release/rollback workflows are
intentionally left unchanged — they reference the signing identity from
prior workflow runs and will need a separate update when the CI signing
infrastructure migrates.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.gitea/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,42 @@
+name: Feature request
+description: Suggest an idea for this project
+title: "[FEATURE]: "
+labels: []
+assignees: []
+body:
+  - type: textarea
+    id: problem-description
+    attributes:
+      label: Is your feature request related to a problem? Please describe.
+      description: "A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]"
+    validations:
+      required: true
+
+  - type: textarea
+    id: desired-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: dropdown
+    id: cli-mode
+    attributes:
+      label: Which CLI mode does this apply to?
+      options:
+        - Both
+        - "npx (@trebuchet/cli)"
+        - "Local (./trebuchet)"
+
+  - type: textarea
+    id: alternatives-considered
+    attributes:
+      label: Describe alternatives you've considered
+      description: A clear and concise description of any alternative solutions or features you've considered.
+
+  - type: textarea
+    id: additional-context
+    attributes:
+      label: Additional context
+      description: Add any other context or screenshots about the feature request here.