docs/setup/mixed-adoption-teams.md: symlink method uses triple-indirection inconsistent with sibling docs

[rbowen]

What should happen

Align the symlink indirection description with the canonical model documented in install-recipes.md and add-a-harness.md (relay symlinks go through .agents/skills/, not through .claude/skills/).

Why

Sibling docs describe a two-level model (canonical in .agents/skills/, relay from .<runtime>/skills/). This doc describes triple-indirection through .claude/skills/, which contradicts the design and confuses contributors adding new harness support.

Which layer

Documentation (docs/, README.md, CONTRIBUTING.md)