Apply a consistent Status/Last-reviewed header convention across core docs

[codeforester]

Only a handful of docs (product-assessment.md, product-requirements.md, repo-command-ownership.md, github-bash-helper-boundary.md, setup-common-ownership.md) carry a Status:/Last reviewed: header. The docs explicitly called out as "Core Documents" in docs/README.md — why-base.md, architecture.md, technical-overview.md, command-reference.md, execution-model.md, runtime-environment.md, tool-boundaries.md, stability-tiers.md — have none. There is no way to tell from the file itself whether these are current without cross-checking against code.

Scope:

• Decide whether the Status/Last-reviewed header convention should be dropped entirely or applied uniformly.
• If applied uniformly, add headers to the listed Core Documents and record the current review date/era.

Acceptance criteria:

• Core Documents either all carry the maintenance header or the convention is explicitly documented as optional/deprecated in docs/README.md's Naming Convention section.