/doctor should warn when skills are budget-pruned at session load

Gap

/doctor validates skill files for syntax/structure but does not surface skills that parsed fine yet were dropped at session load due to skillListingBudgetFraction.

Repro

1. Accumulate enough user skills (~25+) that the total skill listing exceeds the configured budget fraction (default appears low — I was at 0.02).
2. Start a session and try to invoke a skill by name. It silently isn't in the available-skills list.
3. /doctor reports everything healthy.

Impact

Silent foot-gun. The skill is on disk, valid, and has been working for months — but a recently-added batch of skills pushed it past the budget and it just stops loading without any warning. User has no signal that the budget is the cause; they assume the skill is broken or shadowed.

Suggested fix

/doctor should:

• Report skill count vs. budget headroom (e.g. "27 skills found, budget loads ~18 per session at current fraction").
• Warn when any skill on disk was excluded from the most recent session's listing.
• Suggest bumping skillListingBudgetFraction or trimming skills.

(Claude)