Add issue audience conventions for internal vs community issues (#1801)

Documents how we use the `internal` and `help wanted` labels plus
standard footer callouts to signal issue audience.

Author: kmcginnes

docs/agents/issue-audience.md

@@ -0,0 +1,19 @@
+# Issue Audience
+
+How issues in this repo are marked for internal team use vs. open community contribution.
+
+## Internal
+
+- **Label:** `internal`
+- **Footer:**
+
+  > [!IMPORTANT] Internal only — this issue is maintained by the core team and is not accepting external contributions.
+
+## Community
+
+- **Signal labels:** `help wanted`
+- **Footer:**
+
+  > [!IMPORTANT] If you are interested in working on this issue, please leave a comment.
+  >
+  > [!TIP] Please use a 👍 reaction to provide a +1/vote. This helps the community and maintainers prioritize this request.