[diagrams] Add Mermaid diagram to content-pipeline.md

[mccarthy606]

4 of 5 workflow files now have Mermaid diagrams (after v0.2). workflows/content-pipeline.md is the last one without.

What to do

1. Read workflows/content-pipeline.md end-to-end
2. Replace the ASCII pipeline at the top of the file (the "research → draft → record → edit → publish → distribute" line) with a proper Mermaid flowchart LR block
3. Use the same color convention as the other Mermaid diagrams in the repo:
   • manual steps: stroke:#22c55e (green)
   • Claude-assisted steps: stroke:#7c3aed (purple)
   • hybrid steps: stroke:#22d3ee (cyan)
4. Keep the diagram readable on mobile — don't add more than ~10 nodes

Reference

See case-studies/youtube-pipeline.md for the exact same pattern already done — that's the closest sibling.

Effort: ~30 minutes.