fix(cli): treat highlighted install target as selected

[SenLinLeo]

Summary

• Treat the currently highlighted install target as selected when the interactive multiselect prompt submits an empty selection.
• Preserve the existing explicit multiselect behavior; only empty submissions use the highlighted fallback.
• Add a unit test covering the Enter-on-highlighted-target path.

Why

When skillhub install detects multiple install targets, users can move the cursor to a target and press Enter expecting that row to be confirmed. In this flow, prompts can return an empty selection unless the user first checks an
item, so the CLI currently treats it as cancellation.

Falling back to the highlighted target makes Enter behave like confirm while keeping the existing multiselect flow intact.

Tests

• bun test test/unit/agents/resolver-interactive.test.ts test/unit/agents/resolver.test.ts
• bun run typecheck
• bun run lint
• bun run build
• bun test

[CLAassistant]

All committers have signed the CLA.

[SenLinLeo]

It has no impact on historical logic.