Add opt-in SDK logging

[timazed]

Summary

• add a package-level opt-in logging surface with levels, categories, and pluggable sinks
• wire logging through runtime, auth, backend, compaction, and bundled stores
• enable SDK logging from the demo's Developer Logging toggle and improve visible persistence/history diagnostics
• document category filtering, custom AgentLogSink support, and remote telemetry guidance

Logging coverage

• runtime thread lifecycle and messaging
• history append and persistence apply/save paths
• auth restore, sign-in, refresh, and sign-out
• network stream open/fail plus retry decisions
• tool, approval, compaction, and memory activity

Developer integration

• use AgentLoggingConfiguration.console(...) or .osLog(...) for local diagnostics
• provide a custom AgentLogSink to route CodexKit logs into your own telemetry or logging pipeline
• filter by category when you only want focused diagnostics like runtime, persistence, network, or tools
• prefer sinks that buffer or enqueue quickly rather than blocking on network I/O inline

Verification

• swift test
• xcodebuild -project DemoApp/AssistantRuntimeDemoApp.xcodeproj -scheme AssistantRuntimeDemoApp -sdk iphonesimulator -destination 'generic/platform=iOS Simulator' CODE_SIGNING_ALLOWED=NO build