Issue template

One page, fixed fields, bullets only. The same file lives in perch, gitboard and Budgie as .github/ISSUE_TEMPLATE/task.md (a perch test keeps the three copies identical), and /listen and /board write new tasks in it.

Goal: one line, what changes for whom
Done when:
- [ ] checkable outcome
Context:
- one-line fact or constraint
Out of scope:
- one line
Links:
- URL or #iid