feat: add Mastra Code assistant integration

[sergioestebance]

Summary

• Adds Mastra Code as a third supported coding assistant (alongside Claude Code and Gemini)
• Implements full adapter layer: session discovery, activity detection, SQLite database-backed session store, transcript reading, and session migration
• Updates multi-assistant UI (assistant picker, card badges, onboarding wizard) and all related specs

New files

• Sources/KanbanCodeCore/Adapters/Mastracode/ — 5 adapter files (activity detector, database, session discovery, session path, session store)
• Tests/KanbanCodeCoreTests/Mastracode/ — 5 test files
• specs/sessions/mastracode/ — 3 feature specs (activity, discovery, transcript)

Modified

• Domain entities (CodingAssistant) and ports (SessionStore) extended for Mastra Code
• DependencyChecker, HookManager, ImageSender, LaunchSession, EffectHandler updated to handle the new assistant
• UI views updated: ContentView, CardView, CardDetailView, OnboardingWizard, NewTaskDialog, LaunchConfirmationDialog
• Multi-assistant specs and tests updated to include Mastra Code scenarios

Test plan

• swift test passes all existing and new tests
• Verify Mastra Code sessions are discovered on a project that uses it
• Verify assistant picker shows Mastra Code option
• Verify card badges display correctly for Mastra Code sessions
• Verify onboarding wizard includes Mastra Code when installed