refactor(skl): extract skill-listing budget to shared lib (single source of truth)

Chunk 1 of the disableBundledSkills GAP feature. Moves the per-description cap,
aggregate budget constants, calibration note, and the enumerate-and-measure step
out of skill-listing-scanner into scanners/lib/skill-listing-budget.mjs — so SKL
(diagnoses overflow) and the upcoming GAP check (prescribes disableBundledSkills)
consume one budget definition instead of two divergent copies.

- New lib: assessSkillListingBudget (pure aggregate math) + measureActiveSkillListing
  (HOME-scoped enumerate-and-measure wrapper).
- SKL delegates measurement; all finding strings kept byte-identical. 18/18 SKL
  tests pass unchanged → behavior-neutral refactor.
- 12 new lib unit tests pin the budget contract. Suite 875 -> 887.
- README badge + CLAUDE.md test counts synced (self-audit --check-readme: passed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ter3E2JSi1Khgmuf2kady8

README.md

@@ -12,7 +12,7 @@
 ![Commands](https://img.shields.io/badge/commands-18-green)
 ![Agents](https://img.shields.io/badge/agents-6-orange)
 ![Hooks](https://img.shields.io/badge/hooks-4-red)
-![Tests](https://img.shields.io/badge/tests-875+-brightgreen)
+![Tests](https://img.shields.io/badge/tests-887+-brightgreen)
 ![License](https://img.shields.io/badge/license-MIT-lightgrey)
 
 A Claude Code plugin that checks configuration health, suggests context-aware improvements, and auto-fixes issues — `CLAUDE.md`, `settings.json`, hooks, rules, MCP servers, `@imports`, and plugins. 13 deterministic scanners across 10 quality areas, context-aware feature recommendations, auto-fix with backup/rollback, a prompt-cache-aware Token Hotspots scanner with optional API-calibrated `--accurate-tokens` mode, plus cache-prefix stability, dead-tool, and cross-plugin collision detection. Zero external dependencies.