Improve refresh local base ref UX

[brennanb2025]

Summary

• clarify the Refresh Local Base Ref setting around stale git diff main...HEAD / AI diff comparisons
• return a structured local-base-ref refresh result from worktree creation
• show a warning toast when local main/master cannot be fast-forwarded because its owning worktree is dirty

Tests

• pnpm exec vitest run --config config/vitest.config.ts src/main/git/worktree.test.ts src/main/ipc/worktrees.test.ts src/main/runtime/orca-runtime.test.ts src/renderer/src/store/slices/worktrees.test.ts
• pnpm run typecheck:node
• pnpm run typecheck:web
• pnpm run lint