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/skills/arch-checks/SKILL.md

@@ -48,3 +48,11 @@ Instruction/architecture docs (`AGENTS.md`, `CLAUDE.md`, `ARCHITECTURE.md`,
 `docs/architecture/*.md`) must not reference repo file paths that no longer
 exist. If your refactor moved code, update the docs that point at it — the
 error message lists `doc -> stale-path` pairs.
+
+## `check_no_planning_docs.sh`
+
+Planning-type documents must not be committed (see AGENTS.md "Sources of
+Truth"). The guard fails if anything is tracked under `docs/superpowers/` —
+`.gitignore` already ignores it, but `git add -f` bypasses that, so this closes
+the hole. Fix by removing the listed file(s) with `git rm`; keep the plan or
+spec in the issue tracker or a local worktree instead.