fix(setup): fall back when broker auth is busy

[ALV0612]

Summary

• preserve broker RPC errors so auth setup can classify transport failures
• retry auth reads through one direct app-server when a shared broker is busy or stale
• avoid retrying genuine RPC failures and preserve direct failure diagnostics
• add deterministic busy/stale/non-retryable/cleanup coverage

Verification

• focused auth fallback tests: 5/5 pass
• full suite: 96/96 pass
• build and version checks pass
• OpenSpec strict validation and git diff --check pass

Fixes #342

[ALV0612]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

Reviewed commit: 133178d560

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