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.

AGENTS.md

@@ -80,6 +80,9 @@ the repository. Only durable reference — the architecture set under
 and the test-suite references under `docs/testing/` — belongs in version
 control; `.gitignore` ignores everything else under `docs/` by default, so a new
 plan file will not be tracked unless someone force-adds it — don't.
+`scripts/check_no_planning_docs.sh` (wired into `make pre-commit`/`pre-pr` and
+CI) fails the build if anything is committed under `docs/superpowers/`, even via
+`git add -f`.
 
 ## Verification Before PR