docs: add .yardopts so yard and rake doc agree

[tas50]

What

The YARD options lived only in the Rakefile's YardocTask, so a bare yard from the command line produced different documentation than rake doc:

• the Rakefile passed --no-private, a plain yard did not
• neither had a --readme, a --title, or an extra-file list

This adds .yardopts as the single source of truth and lets the YardocTask pick it up, matching the other test-kitchen drivers. The redundant lib/**/*.rb glob is dropped from :doc_coverage so that task reads .yardopts too.

Note on --private

Switching from --no-private to --private/--protected widens the surface YARD reports on from 9 methods to 23. All 23 are already documented, so coverage stays at 100% — this just stops hiding two thirds of the driver from the generated docs.

Verification

yard and yard stats --list-undoc now return identical output:

Files:           2
Modules:         2 (    0 undocumented)
Classes:         1 (    0 undocumented)
Constants:      10 (    0 undocumented)
Attributes:      0 (    0 undocumented)
Methods:        23 (    0 undocumented)
 100.00% documented

rake -T still lists both tasks:

rake doc              # Generate YARD Documentation
rake doc_coverage     # List anything in lib/ that is still undocumented

Docs-only change — no lib/ or spec/ files touched.

🤖 Generated with Claude Code