Inject provider-declared credentials into auth preflight

[rudijetson]

Problem

rudi agent hosts checks provider authentication with only the executable PATH environment. When a provider credential exists in RUDI managed secret storage, the actual launch receives it but the preflight probe does not, so readiness can be reported incorrectly.

Required behavior

• build the provider environment before version and authentication probes
• inject only credentials declared by the provider contract
• preserve executable PATH construction
• never forward unrelated stored secrets
• cover the behavior with a temporary isolated RUDI home

Scope

This issue extracts the existing follower-Mac preflight change into an isolated branch. It excludes the dirty primary checkout’s unrelated CLI, provider ownership, shim, CRM, and generated-artifact changes.