chore(ci): guard against committed planning docs and remove re-added ones (#4777)

chore(ci): guard against committed planning docs; remove re-added ones

PR #4771 removed the docs/superpowers planning-doc archive and added a rule,
but #4765 force-added two more (git add -f bypasses .gitignore):
docs/superpowers/plans/2026-07-12-observability-single-writer.md and
docs/superpowers/specs/2026-07-12-observability-single-writer-design.md — an
agentic implementation plan and its design spec. Delete both.

Close the enforcement gap so this cannot recur:
- New scripts/check_no_planning_docs.sh fails if anything is tracked under
  docs/superpowers/, regardless of how it was added.
- Wire it into make pre-commit/pre-pr/dev-check.
- Run it in CI: ci.yml for code/mixed PRs, and ci-docs-only.yml (which was a
  green stub) so docs-only PRs can no longer slip a planning doc past the
  required 'Test and Lint' check.
- Document the guard in AGENTS.md and the arch-checks skill.

.github/workflows/ci.yml

@@ -134,6 +134,9 @@ jobs:
       - name: Check body-cache whitelist guard
         run: ./scripts/check_body_cache_whitelist.sh
 
+      - name: Check no planning docs committed
+        run: ./scripts/check_no_planning_docs.sh
+
   test-and-lint:
     name: Test and Lint
     if: github.event_name != 'pull_request' || github.event.action != 'closed'