docs: advance to Public Preview

[tensornull]

Summary

• advance the repository contract from completed Private Remote Validation to the pre-visibility Public Preview phase
• remove stale first-push and private-repository initialization instructions while preserving the mandatory visibility-change stop
• mark Public Preview in progress and record Dependabot PR chore(deps): bump actions/checkout from 4.2.2 to 7.0.1 #2 as deferred from the pre-visibility merge set

PR #2 evidence

Dependabot PR #2 proposes a major actions/checkout update from 4.2.2 to 7.0.1. Its credential-free CI failed because the mutable-action-reference regression test hard-codes the previous checkout SHA; all test lanes failed on that assertion and downstream artifact and copied-checkout jobs skipped. The roadmap now requires version-independent coverage and a completely successful replacement CI run before reconsidering the update.

Validation

• complete local non-live test suite: 116 passed, 1 deselected
• Ruff lint and format checks; Pyright with 0 errors
• version, changelog, Public Preview content, secret, and workflow semantic gates
• checksum-pinned actionlint 1.7.12 from verified offline cache
• clean wheel and sdist build, Twine and artifact checks
• independent wheel and sdist clean-install smokes
• complete copied standalone repository verification on the final documentation

No visibility, secret, environment, live API, release, or publication operation was performed.