Implement /insights session-history analytics and reporting

[yaohaowei0914]

Background

Rust currently lacks /insights. The Bun reference scans local session history, aggregates stats/facets, and writes a report.

Reference

• Entry: src/commands/insights.ts
• Related modules: session file scanning, log loading, facet extraction, queryWithModel-driven section generation.
• Behavior: deduplicate/filter session history, extract facets, generate a report, and return a stable report location.

Rust Gap

• No /insights command today.
• Rust has session persistence/export foundations but no analytics/reporting service.
• Suggested approach: create a session-analytics service first, then add a thin slash-command shell.

Acceptance

• Scan session history and extract basic statistics/facets.
• Produce a structured report, at least in terminal/text form and ideally with a richer artifact later.
• Filter minimal/meta sessions appropriately.
• Add analytics-service tests.