InstallerEngine: Implement WriteConfig recipe

[malpern]

Problem

WriteConfig recipe throws "not yet implemented" error, though config deployment works through other paths.

Location

InstallerEngine.swift lines 486-490

private func executeWriteConfig(_ recipe: ServiceRecipe, ...) async throws {
    throw InstallerError.unknownRecipe("writeConfig recipe not yet implemented")
}

Impact

Automated config deployment via installer recipes is broken, but workarounds exist.

Fix

1. Implement executeWriteConfig method
2. Wire into recipe execution flow
3. Test automated deployment

References

• Comprehensive review: docs/code-review/comprehensive-feature-review.md section 6.2