Scan skill roots concurrently

[copyberry]

Scan skill roots concurrently

What changed

• Scan up to eight skill roots concurrently instead of loading them one at a time.
• Share scan capacity across host-skill and plugin-skill loads to bound aggregate filesystem work.
• Merge completed scans in input order so root precedence and error ordering remain deterministic.
• Skip canonicalization and parsing work for empty roots.

Testing

• Add coverage for out-of-order scan completion and shared-capacity blocking.