archive-sprintlisted

Archive the completed sprint for backlog item #$ARGUMENTS. ## Steps 1. **Find the planning folder**: Look in `docs/planning/` for the directory corresponding to backlog item #$ARGUMENTS. 2. **Verify completion**: Check that the work is actually done: - Read the sprint plan and verify tasks are complete - Verify the Sprint Report section is filled in (Results, Metrics, What went well/wrong, Learnings, Unresolved). If missing, tell the user to write the post-sprint report first — do not archive without it. - Check if any new learnings should be added to `docs/reference/post-sprint-learnings.md`. If the sprint report has learnings not already captured there, append them. - **Run the project's checks and confirm they actually pass** — the test suite, linter, and build, executed now (not assumed from the report). Do not archive on a red or unrun check; capture the real result. A "green" claim must cite the command + output (`.claude/rules/quality-gates.md` §2.5). 3. **Move to archive**: Use `git mv` to move: - The scope doc → `docs/archive/plans/{slug}-scope.md` - The sprint plan → `docs/archive/sprints/{slug}-sprint.md` - Any other planning docs in the folder → `docs/archive/plans/` 4. **Update backlog**: Find the item in its horizon file (`docs/backlog/now.md`, `next.md`, or `later.md`) and move it to `docs/backlog/completed.md` with `Completed` status and the date. Update the item counts in `docs/backlog/README.md`. 5. **Clean up**: Remove the now-emp