Share the skills budget across host and executor catalogs

[copyberry]

Share the skills budget across host and executor catalogs

Why

Rendering host and executor skill catalogs independently allows their combined
metadata to exceed the skills context budget.

What changed

• Allocate one budget across both catalogs when both contain model-visible
  skills.
• Prefer retaining executor skills when budget pressure requires omissions,
  while preserving an executor omission marker and reporting when all host
  skills were omitted.
• Evaluate absolute and aliased host paths under the shared budget and select
  the rendering that best preserves executor entries, total entries, and
  descriptions, in that order.

Testing

Added renderer and production-turn coverage for shared-budget description
shortening, host-first omission, executor omission markers, and alias selection.