Reject ignored shipped-baseline opt-in in recursive multi-skill scans #822

When a directory contains child skills, scan --recursive --use-shipped-baseline enters multi-skill dispatch without applying or rejecting the shipped-baseline option. The same child scanned directly applies its shipped baseline. This also changes --strict outcomes unexpectedly.

Reproduced on main b07f181d9138878c3827de0947a9fd2c6a84a66c with synthetic local fixtures and --no-llm:

skillspector scan collection/one --no-llm --format json --use-shipped-baseline
skillspector scan collection --recursive --no-llm --format json --use-shipped-baseline

With a reviewed rule baseline in each child's .skillspector-baseline.yaml, the first command suppresses the matching finding; the second leaves it active and reports no suppressed findings. A malformed shipped baseline likewise fails the direct opted-in scan but is ignored by recursive multi-skill dispatch.

Proposed fix: reject --use-shipped-baseline with exit 2 when dispatching recursive multi-skill scans and tell the caller to scan each child separately. This follows the existing explicit --baseline behavior discussed in #201 and #310. Keep single-root skill scans with --recursive supported, and keep default recursive scans without the opt-in unchanged. Per-child baseline application is outside this fix.

Regression coverage should include one or more detected children, single-root skills, explicit-baseline precedence, machine-readable output, and preservation of an existing --output file on the argument error.