feat(desktop): enrich agent context and improve diff UX

[jouwdan]

Summary

• Enrich agent system prompts with live IDE context: active editor file, selected diff file, terminal status, dev server URL, console errors, git summary, and project instruction files.
• Add instruction precedence rules for app safety, tool docs, user requests, and repo instructions.
• Persist selected diff file state on workspace tabs.
• Refresh visible git diff UI more aggressively and ignore stale diff responses.
• Make split-pane focus update when clicking inside a pane and highlight the focused active tab in orange.
• Open git diff in the right pane by default, creating split view when needed.

Testing

• pnpm --filter @meith/shared test
• pnpm --filter @meith/shared typecheck
• pnpm --filter @meith/protocol typecheck
• pnpm --filter @meith/desktop exec vitest run src/main/__tests__/agent.test.ts src/main/__tests__/spaces.test.ts src/main/__tests__/storage.test.ts src/main/__tests__/integration.test.ts
• pnpm exec biome check ...
• git diff --check