feat: gap reports as shareable PDFs

[rajnavakoti]

Description

Export eval results and gap analysis as a formatted PDF report that can be shared with stakeholders who don't use the CLI or viewer. The report should be self-contained — no need to run CB to understand the findings.

Acceptance Criteria

• cb export report --format pdf --output ./report.pdf generates a PDF
• Report includes: executive summary, coverage scores by persona, gap breakdown (CLEAN/INCOMPLETE/MISSING), top 10 gaps with descriptions, entity type distribution chart
• Branded but minimal design (no heavy dependencies)
• Can be generated from eval results JSON without re-running eval
• Under 10 pages for typical KB

Out of Scope

• Custom branding/theming
• Interactive PDF elements