docs: first-party-cutover.md isn't linked from any other doc

[codeforester]

Location and audience

docs/first-party-cutover.md, reviewed against origin/main (8de2056,
"security: constrain standalone application payloads (#520)") during a
pre-release documentation sweep (2026-09-21). Audience: a maintainer or new
contributor trying to understand the v1.4.0-to-v2 first-party cutover history,
or anyone auditing what first-party-cutover.yaml / scripts/first-party-cutover
are for.

The 44-line doc (Issue #240's promotion-handoff record) is not linked from any
other tracked markdown file in the repo — not README.md's "Documentation
map" section, not docs/README.md, not CHANGELOG.md. The only repo hits for
"first-party-cutover" are the sibling first-party-cutover.yaml and
scripts/first-party-cutover, which are different files with a similar name.
It's discoverable only by browsing docs/ directly.

Suggested correction

Either:

• add one line linking docs/first-party-cutover.md from README.md's
  "Documentation map" section (it fits near the versioning-policy /
  release-process entries as historical migration record), or
• if it's intentionally archival now that the v2 cutover (Migrate first-party consumers and Homebrew to the Base Bash 2.0 contract #240) is complete,
  say so explicitly at the top of the doc and/or move it under a clearly
  historical section, so its unlinked status reads as a decision rather than
  an oversight.