feat: health check packaging — cb health-check command

[rajnavakoti]

Description

Package the extract + eval + gap report flow into a single cb health-check command. One command that takes a folder of docs and produces a complete knowledge health report. This is the "free health check" offered to conference attendees.

Acceptance Criteria

• cb health-check ./docs/ --output ./report/ runs full pipeline: extract → eval → report
• Produces: entity files, eval results JSON, gap report markdown, summary statistics
• Summary shows: total entities, coverage score, top 5 gaps, persona breakdown
• Can be run without prior cb init (creates a temporary block)
• Total cost estimate shown at the end
• Exit code 0 if coverage > threshold (configurable), 1 otherwise

Out of Scope

• PDF report generation (separate issue)
• Hosted health check service