fix: address code review findings across installer, auth, and brew

[fullstackjam]

Summary

• installer — dead step* functions removed: `stepMacOS`, `stepPostInstall`, `stepShell`, `stepDotfiles`, `hasDotfiles` had no production callers; `Apply()` uses the `apply*` variants exclusively. The 14 `TestStep*` and 5 `TestHasDotfiles*` tests were exercising dead code, providing false coverage confidence.
• installer — real coverage added: replaced the removed tests with equivalent tests on `applyShell`, `applyDotfiles`, and `Plan()` entry points.
• auth — httputil bypass fixed: `pollOnce` was calling `httpClient.Get()` directly, skipping HTTP 429 + Retry-After handling. Now uses `httputil.Do()` consistently with `startAuthSession` in the same file.
• brew — error propagation: `InstallWithProgress` now returns a non-nil error when packages permanently fail after retries (previously swallowed failures silently).
• brew — context threading: `ctx context.Context` added as first parameter to `InstallWithProgress` and all internal brew helpers; `exec.CommandContext` used so brew subprocesses are cancellable.
• brew — retry deduplication: `retryBrew()` helper extracted, eliminating duplicated retry-with-backoff boilerplate between `installFormulaWithError` and `installSmartCaskWithError`.
• brew — truncation limit: `parseBrewError` default line truncation increased from 60 → 120 chars.
• brew — disk estimation: `PreInstallChecks` now takes `(formulaeCount, caskCount int)` and uses 0.1 GB/formula vs 0.5 GB/cask instead of a flat 0.2 GB/pkg.
• archtest baselines regenerated to reflect removed dead code and shifted line numbers.

Test plan

• `go test ./internal/...` — all 21 packages pass
• `go vet ./...` — clean
• Archtest baseline regenerated with `ARCHTEST_UPDATE_BASELINE=1`