feat(skill-quality): read the description target and warn thresholds from settings #11971

Refs #7582

Blocked by: #11954
Blocked by: #11965

Outcome

The description standard is written in the skill criteria, and its thresholds are settings with named defaults.

Decisions

• D82, Q183, Q184: one what-sentence, "Use when" with one trigger per branch, at most three probe-proven hook terms. Target 250, warn above 419, fail at 1,024. Settings: description target 250; warn 419 (1,024 fixed).

Done when

• Skill criteria state the description standard: one what-sentence, "Use when" with one trigger per branch, and at most three probe-proven hook terms.
• skill-quality:check reads the description target (default 250) and the warn threshold (default 419) as settings resolved through the convention home that config-cascade: settings resolution and convention home #7571 delivered, and fails at 1,024 characters as a fixed limit. A test changes the warn setting and observes the warning move.

Out of scope

• Trimming existing descriptions (perf(skills): trim skill descriptions until the listing fits, then lower skillListingBudgetFraction #6846, Concise skills: one skill-creation standard, evals, and a fleet-wide rollout #6476).