feat: glob-based copy_to_workspace for fixture seeding

[devzeebo]

Summary

• Adds copy_to_workspace(workspace, globs, options?) so specs can seed the temp workspace from fixture globs instead of one-off writeFile calls (#6).
• Globs resolve from the current Vitest spec directory (overridable with from); optional base strips a matching path prefix. Empty globs and base mismatches throw before copying.
• Documents the helper in the README and covers preserve-structure, strip-base, multi-glob, nested dirs, errors, and default from.

Test plan

• pnpm test — unit suite including copy_to_workspace cases
• Confirm a given step can copy fixtures/** into this.workspace without passing from
• Confirm base: "fixtures/*" drops the first two path segments
• Confirm a non-matching glob and a non-matching base throw and leave the workspace empty

[devzeebo]

Requesting changes from review: destination collisions are skipped silently, from is cwd-resolved (README example is misleading), empty globs is a no-op, destinations are not confined to workspace, Vitest is a static optional-peer import, and glob expansion walks the entire from tree. Error tests also do not assert that nothing was copied. Fixing these on the branch next.