Usage Monitor Architecture

UsageMonitorService (MainActor, Observable)
    │
    ├── CredentialManager (Actor)
    │   └── Loads credentials from auth.json, env vars, keychain
    │
    └── Providers (Actors)
        ├── AnthropicProvider
        ├── CopilotProvider
        ├── GeminiCliProvider
        ├── AntigravityProvider
        └── SyntheticProvider

@MainActor
@Observable
final class UsageMonitorService {
    var snapshots: [AIProvider: UsageSnapshot]
    var isRefreshing: Bool
    var enabledProviders: Set<AIProvider>
    
    func startMonitoring()
    func stopMonitoring()
    func refreshAll() async
}

actor CredentialManager {
    func credentials(for provider: AIProvider) -> [String: Any]?
    func accessToken(for provider: AIProvider) -> String?
    func hasCredentials(for provider: AIProvider) -> Bool
}

protocol UsageProvider: Sendable {
    var id: AIProvider { get }
    var displayName: String { get }
    
    func hasCredentials() async -> Bool
    func fetchUsage() async throws -> UsageSnapshot
}

struct UsageSnapshot {
    let provider: AIProvider
    let displayName: String
    let windows: [RateWindow]
    let lastUpdated: Date
    let error: UsageError?
    
    // Provider-specific extras
    let extraUsageEnabled: Bool?  // Anthropic
    let requestsRemaining: Int?   // Copilot
}

struct RateWindow {
    let label: String           // "5h", "Month", "Pro"
    let usedPercent: Double     // 0-100
    let resetDescription: String?
    let resetAt: Date?
}

┌─────────────────────────────────────────────────────┐
│                 Credential Loading                   │
├─────────────────────────────────────────────────────┤
│  1. ~/.pi/agent/auth.json (primary)                 │
│     └── Keys: anthropic, github-copilot,            │
│         google-gemini-cli, google-antigravity,      │
│         synthetic                                    │
│                                                      │
│  2. Provider-specific fallbacks:                    │
│     ├── Anthropic: macOS Keychain                   │
│     ├── Copilot: ~/.config/github-copilot/hosts.json│
│     ├── Gemini: ~/.gemini/oauth_creds.json          │
│     └── Synthetic: SYNTHETIC_API_KEY env var        │
└─────────────────────────────────────────────────────┘