Add first-class Markdown viewing and import

[dudash]

Summary

• Add a shared Markdown renderer plus mdm render and pretty-by-default mdm changelog output.
• Add read-only ordinary Markdown handling in mdmind, including rendered/raw views, search highlighting, near-miss recovery, and explicit --as markdown / --as map overrides.
• Add TUI Markdown import into -mind.md with immediate open, collision handling, timestamped lossy summary, text-preservation counts, link-only bullet fixes, and fenced-code preservation.
• Document the PRD and update user/agent/release docs for the new Markdown viewing behavior.

Linear

• MDM-59
• MDM-60
• MDM-61

Validation

• cargo test
• python3 scripts/docs/check-doc-links.py
• git diff --check
• Manual README import check to /private/tmp/mdmind-readme-import-check.md, confirming fenced mdmind examples stay literal detail lines rather than becoming child nodes.