Add iOS platform support, enable App Group snapshot storage on Darwin, and constrain widget providers for iOS MVP

[waffensam]

Motivation

• Prepare the package and widget code to support an iOS companion app and Home Screen widgets.
• Allow the widget snapshot persistence to use the App Group container on Darwin platforms (so iOS widgets/apps can share data).
• Provide an iOS MVP that exposes a limited set of providers for widgets to simplify initial delivery.

Description

• Add .iOS(.v17) to Package.swift platforms to enable building for iOS.
• Change WidgetSnapshotStore.snapshotURL to use #if canImport(Darwin) so App Group container URLs are considered on Darwin platforms (including iOS).
• Introduce WidgetProviderScope to limit available widget UsageProviders to .codex and .claude for iOS, and add fallbackProvider for safe fallback behavior.
• Gate additional ProviderChoice cases and initialization with #if !os(iOS) so only the intended provider choices are exposed on iOS, and make supportedProviders(from:) fall back to WidgetProviderScope.fallbackProvider when none are supported.
• Update CodexBarWidgetProvider logic and tests in Tests/CodexBarTests/CodexBarWidgetProviderTests.swift to reflect the iOS-limited provider set and platform-conditional expectations.
• Add docs/IOS_MIGRATION.md documenting the phase-1 iOS migration plan and rationale.

Testing

• Ran the unit test file Tests/CodexBarTests/CodexBarWidgetProviderTests.swift via swift test and the updated tests passed.
• The adjusted provider selection behavior is covered by the updated tests which succeeded.

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Codex Task