chore: remove internal architecture docs from public .github

[pdodgen-revparts]

Summary

Removes four internal architecture documents that were committed to this public repo during DEVEX-1628 and DEVEX-1630 work:

• docs/superpowers/plans/2026-06-08-unify-php-build-workflows.md
• docs/superpowers/plans/2026-06-10-common-bump-build-on-nightly.md
• docs/superpowers/specs/2026-06-08-unify-php-build-workflows-design.md
• docs/superpowers/specs/2026-06-10-common-bump-build-on-nightly-design.md

These files contain internal CI architecture details, specific service repo names, secret names (REPO_WRITE_PAT), and direct line-anchored references to workflow files. Not credentials, but legitimately internal — they shouldn't live on a public repo.

Approach: delete from main only

History retains the deleted files. We chose this over full history rewrite because:

• Content isn't credentials, just architecture detail
• Force-pushing main here requires temporarily disabling branch protection (cloudops/admin)
• History rewrite is also incomplete: GitHub retains deleted commits by SHA for ~90 days, the existing fork retains history, third-party crawlers may have already indexed
• The practical exposure surface drops massively when files no longer appear in tree / search / repo readme — that's the realistic win

If a stronger erasure is wanted later (e.g. cred-level scrub), git filter-repo --invert-paths --path docs/superpowers/ + force-push is the followup, but should be done as a separate explicit decision.

Going forward

Internal architecture docs should land in private repos, not here. Going to recommend on DEVEX-1659 that future plans live in the (now-renamed) encodium/actions private repo, or in a dedicated internal-docs repo.

[cursor]

PR Summary

Low Risk
Documentation-only deletion with no runtime or workflow impact; only risk is losing in-repo copies of those guides unless they live elsewhere.

Overview
Deletes four internal DEVEX architecture documents from docs/superpowers/ — two implementation plans and two design specs covering PHP build workflow unification (DEVEX-1630) and nightly common-bump CI behavior (DEVEX-1628).

No workflow, code, or config files change; only markdown is removed from the default tree so internal repo names, secret identifiers, and CI rollout detail are no longer browsable on this public repository.

^{Reviewed by Cursor Bugbot for commit 7c61545. Bugbot is set up for automated code reviews on this repo. Configure here.}