feat: Markdown incident report export for explain

[leo-aa88]

Summary

Provide a markdown incident report suitable for postmortems and tickets (not only a raw CLI markdown dump).

Motivation

• README Roadmap lists markdown incident report export.
• CLI already documents raglogs explain --format markdown; this issue covers a complete report shape (sections, metadata, optional links) if not already satisfied.

Scope (proposal)

• Audit current explain --format markdown output; extend with title, window, services, evidence bullets, confidence, and optional “how to reproduce query” footer.
• Document raglogs explain ... > postmortem.md in README.
• If JSON is the source of truth, consider a single template layer shared by text/md.

Acceptance criteria

• One command produces a paste-ready .md file for a typical incident.
• Snapshot or unit tests for markdown structure.

Notes

• Close or narrow this issue if current markdown already meets the bar; then update the roadmap wording.