Document Apple companion install paths

[gaelic-ghost]

Summary

• document standalone apple-dev-skills behavior versus the productivity-skills companion requirement
• add socket marketplace guidance for users who want companion plugins installed together
• improve missing maintain-project-repo failure messages in Apple bootstrap and guidance-sync workflows
• bump maintained version surfaces to 6.1.1 for the patch release

Subtree accounting

• apple-dev-skills: pushed out to gaelic-ghost/apple-dev-skills main and released as v6.1.1
• SpeakSwiftlyServer: not touched

Verification

• bash .github/scripts/validate_repo_docs.sh
• uv run pytest in plugins/apple-dev-skills
• uv run scripts/validate_socket_metadata.py
• git diff --check