Remove stale design-spec references and fix doc-gen wording

[jeffhorn-nava]

What

Removes references to a docs/superpowers/ design spec that was never committed, and corrects README's description of when the doc-generation Actions run. The pipeline and skills are themselves the source of truth now, so the historical spec does not need to live in the repo.

Closes #2

How

Dropped all six sites that pointed at the spec: the docs/superpowers/ links in README.md, AGENTS.md, and rules/architecture.md, and the §-section annotations (§3.3, §3.3b, §3.6, §8) in scripts/lint_docs.py, skills/generate-strata-docs/SKILL.md, and skills/generate-strata-docs/references/agents/source-doc.md. Load-bearing phrasing next to an annotation (e.g. "no fallback" beside §8) was preserved. Also changed README's "manual, weekly, or on sources.md change" to "manual", matching the workflow_dispatch-only triggers and AGENTS.

Test plan

• python -m scripts.lint_manifest → MANIFEST_OK (9 sources)
• python -m scripts.lint_docs → DOCS_OK (56 docs)
• python -m scripts.build_graph → GRAPH_OK; no change to docs/INDEX.md or docs/graph.json
• python -m pytest -q → 53 passed
• grep confirms no docs/superpowers or spec § references remain

Notes for reviewers

Docs, skill instructions, and code comments only; no build or runtime impact. The FMLA legal citations (§ 2612, § 825) in docs/sources/ and docs/.verification/ are unrelated to the design spec and were left in place.

[jeffhorn-nava]

Adjacent correction to fold in: stale trigger comment in generate-docs.yml

While reviewing this PR (which correctly narrows the README trigger wording to "manual"), I found one more spot with the same class of stale claim, just outside this diff. Flagging it here so it can be picked up separately.

File: .github/workflows/generate-docs.yml, line 3

Current:

# Routine drift refresh + new-source pickup lives in update-docs.yml (cron + manual).

Problem: update-docs.yml is on: workflow_dispatch: only — there is no schedule/cron trigger. The "(cron + manual)" parenthetical contradicts both the actual workflow definition and rules/ci.md:15, which already states both workflows are "manual only."

Suggested fix: drop the cron + , e.g.

# Routine drift refresh + new-source pickup lives in update-docs.yml (manual dispatch).

Not introduced by this PR, so leaving it out of scope is fine. Noting it because it's the same "when do the doc-gen Actions run" accuracy fix this PR makes, in the place a maintainer is most likely to read it.