Per-project default AI provider

[Ron537]

Summary

Allow each project to have its own preferred AI provider, overriding the global default. When the user clicks "New AI session" on a project, DPlex starts the session with that project's preferred provider.

Acceptance criteria

• Extend Project with defaultProviderId?: string.
• When set, startAISession(project) honors it (currently uses the global defaultAITool setting).
• UI: dropdown in the project's right-click menu → "Default AI tool for this project" → list of registered providers, with "Use global default" as the first option.
• Persists in settings.json alongside other project metadata.
• Falls back to global default when the per-project provider is no longer registered (e.g. provider removed).

Files to touch

• src/renderer/src/types/index.ts (extend Project)
• src/renderer/src/stores/projectStore.ts (startAISession, persistence)
• src/renderer/src/components/projects/ProjectItem.tsx (menu UI)
• tests/unit/project-store-active.test.ts (cover the override)

How to test

• Create two projects, set one to Copilot CLI and another to Claude Code, leaving global default alone.
• "New AI session" on each → expect the right tool starts.

Effort: 1 day.