…nt-tooling bootstrap

Users rely on gx status and gx setup to validate and provision the required global toolchain.
This change adds @imdeadpool/codex-account-switcher to the tracked global package set
so status surfaces it and setup/init install it whenever it is missing.

The setup checklist/help copy and README references were updated to keep the documented
install command aligned with runtime behavior, and tests now cover the expanded package
set and missing-package install path.

Constraint: Setup/install behavior must remain backward compatible with existing OMX/OpenSpec checks
Rejected: Add a separate one-off codex-auth install command | would duplicate setup responsibilities and drift from status detection
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep GLOBAL_TOOLCHAIN_PACKAGES and setup/checklist docs in sync whenever global dependencies change
Tested: npm test (49/49); node --check bin/multiagent-safety.js
Not-tested: Real networked npm global install path on a machine missing codex-account-switcher