Add persona and team import polish

[klopez4212]

Summary

• Treats markdown persona files as single-item imports for file picker and drag/drop flows.
• Preserves provider and name-pool metadata through batch import and import-update helpers.
• Adds focused helper tests and closes the team dialog after a successful directory install.

Notes

• This standalone PR intentionally avoids the avatar-ref helper from the avatar plumbing PR.

Validation

• cd desktop && pnpm check
• cd desktop && pnpm typecheck
• node --import ./test-loader.mjs --experimental-strip-types --test src/features/agents/ui/batchImportPersonaInput.test.mjs src/features/agents/ui/personaImportPlan.test.mjs src/features/agents/ui/personaImportUpdateInput.test.mjs src/shared/lib/fileMagic.test.mjs
• git diff --check