docs: require branch/tag ref field on all issues in contributing guidelines

Added requirement that every issue must have its Forgejo branch/tag (Ref) field
set to the working branch once work begins. This field is separate from the
branch name in the issue body Metadata section but must match it. Setting the
field allows Forgejo to associate issues with branches in its UI.

Refs: #408

CONTRIBUTING.md

@@ -615,6 +615,12 @@ Every issue must include:
   `State/In Progress`, `State/Paused`, `State/In Review`, or `State/Completed`), a milestone
   is mandatory. If no appropriate milestone exists, discuss with the project owner before
   proceeding.
+- **Branch/Tag (Ref):** Every issue must have its Forgejo **branch/tag** field set to the
+  branch where work for the issue is being done. This is the `Ref` field on the issue form in
+  Forgejo — it is separate from the branch name documented in the issue body's Metadata
+  section, although both should match. Setting this field allows Forgejo to associate the
+  issue with the correct branch in its UI and link tracking. The field should be set when
+  work begins on the issue (i.e., when it moves to `State/In Progress` or later).
 - **Description:** The body of the issue must contain the following sections:
   - **Background and context:** Why does this issue exist? What is the motivation?
   - **Current behavior** (for bugs): What is happening now that is incorrect or undesirable?