Docs: Homebrew/source-checkout install recipes duplicated 3x and already drifted

[codeforester]

Goal

Consolidate the Homebrew and source-checkout install recipes, which are
currently written out three separate times each and have already drifted
out of sync with each other.

Background

A documentation review found:

• The Homebrew install recipe (brew trust / brew install / basectl setup / ...) is written three separate times: README.md:144-149
  ("Start Here"), README.md:1433-1441, and FAQ.md:78-84. The copies have
  already drifted — the "Start Here" version omits the basectl update-profile and exec "$SHELL" -l steps that both other copies
  include, so the "identical" instructions already disagree on completeness.
• The source-checkout install recipe shows the same pattern:
  README.md:151-155, README.md:1523-1530, and FAQ.md:98-103 — three
  near-duplicate blocks (one shorter) that each require a synchronized edit
  if install steps ever change.
• The same Homebrew quick-start is also restated verbatim in
  docs/bootstrap.md:163-169 and twice more in
  docs/macos-install-validation.md:92-98,109-113, with no "see
  bootstrap.md" pointer anywhere.

Scope

• README.md
• FAQ.md
• docs/bootstrap.md
• docs/macos-install-validation.md

Acceptance Criteria

• One canonical copy of the Homebrew install recipe and one canonical
  copy of the source-checkout recipe exist (a good candidate location is
  README.md's "Start Here" section and/or docs/bootstrap.md).
• All other locations either link to the canonical copy or are replaced
  with a short pointer instead of a restated block.
• The canonical recipe includes the complete step sequence (including
  basectl update-profile and exec "$SHELL" -l) so no copy is missing
  steps.

Validation

git diff --check; grep for the install command sequence across the repo to
confirm only one full copy remains (plus cross-references).

Non-Goals

• Changing what the install steps actually do — this is a
  duplication/drift fix, not a behavior change.

Project Metadata

• Priority: P2
• Size: S
• Area: Docs
• Initiative: Adoption Polish
• Agent Assignment: Copilot candidate after triage (mechanical
  de-duplication with a clear canonical source to point to)