[codex] remove obsolete System Book design

[divo12]

Summary

The repository still tracked an approved System Book design even though current OpenFlywheel evolution work no longer uses that direction. Keeping it made an obsolete design available as an apparent product input, research authority, alternative, or active specification.

This PR deletes the obsolete tracked design. No replacement wording was needed because tracked-file searches found no directly related references outside the deleted document.

Root cause and effect

The legacy design document remained tracked after the fresh branch moved to the current evolution model. Readers and agents could therefore treat its product boundary, research basis, and acceptance criteria as current guidance.

Validation

Local evidence:

• Confirmed the branch starts at exact origin/fresh SHA 1d91d03402499377a6de97aff5e81323d1dc0747.
• git diff --check passed before and after staging.
• Tracked/index grep found no remaining System Book or obsolete spec-path references.
• Staged and committed diff review showed exactly one file deletion and no added or modified files.
• Working tree was clean after commit.

Source, test, and plugin suites were not run because this change only deletes an unreferenced Markdown specification. The local pre-push hook also reported no supported checks and skipped pytest because it was not installed in that hook environment.

---

Summary by cubic

Removes the obsolete System Book design so it no longer reads as current product direction.

The document was approved before the fresh branch moved to the current evolution model, and a tracked-file search found no references to it elsewhere. This change only deletes the markdown file; no replacement wording was needed.

^{Written for commit 1c3996e. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}