Add MiniMax provider support

[kbdevs]

Summary

• Add MiniMax as a first-class API-key provider in Settings
• Store minimax-*.json auth files and include them in config hot-reload fingerprints
• Inject a managed OpenAI-compatible MiniMax provider using https://api.minimax.io/v1 with default MiniMax model aliases
• Update provider wiring tests, verification specs, README, and Factory setup docs

MiniMax OpenAI-compatible API reference: https://platform.minimax.io/docs/api-reference/text-openai-api

Testing

• cd src && swift test
• make test
• swiftc Sources/ProviderCatalog.swift Sources/CustomProviders.swift Sources/ConfigComposer.swift Verification/ConfigComposerSpec.swift -o /tmp/ConfigComposerSpec && /tmp/ConfigComposerSpec
• swiftc Sources/ProviderCatalog.swift Sources/CustomProviders.swift Sources/ConfigComposer.swift Sources/ZAIAPIKeyStore.swift Verification/ZAIAPIKeyStoreSpec.swift -o /tmp/ZAIAPIKeyStoreSpec && /tmp/ZAIAPIKeyStoreSpec
• swiftc Sources/ConfigInputFingerprint.swift Verification/ConfigInputFingerprintSpec.swift -o /tmp/ConfigInputFingerprintSpec && /tmp/ConfigInputFingerprintSpec
• git diff --check

Note: swift test still emits the existing AppDelegate.windowDidClose / windowDidExpose warning unrelated to this change.