[codex] Track Codex usage and reduce repeated context

[jamesrochabrun]

Summary

• records provider-reported Codex token usage from turn.completed events and persists session/workspace totals
• shows token usage only in DEBUG builds for now, so production users do not see confusing API-token totals
• stops resending Codex developer instructions and Easel runtime hidden context on follow-up turns
• adds a compact resource manifest delta hidden context for follow-up messages when project resources are added, updated, or removed
• removes the visible return / shift-return composer hints

Why

The token badge exposed that small follow-up messages could still report very large API-token totals. Part of that was expected provider accounting, but we were also adding repeated hidden context that did not need to be resent on Codex resume turns. This PR keeps exact provider usage available for development while reducing repeated prompt payload where Easel controls it.

Validation

• swift test in Packages/EaselClaudeCodeUI passed 124 tests
• xcodebuild test -scheme EaselChat-Package -destination 'platform=macOS' -only-testing:EaselChatTests/EaselAgentInstructionsTests -only-testing:EaselChatTests/ChatServiceTests passed 18 focused tests
• xcodebuild build -project Easel.xcodeproj -scheme Easel -configuration Release -destination 'platform=macOS' exited successfully
• git diff --check clean

Note: Xcode logs still print the existing third-party SwiftLint plugin messages about missing Output folders for CodeEdit packages, even when the build/test command exits successfully.