docs(diagram): refresh legacy architecture diagram for 10 adapters

The original docs/diagram.svg was stuck at 9 harness boxes and was
missing Codex entirely. README now says 10 adapters everywhere, so
this one kept contradicting the rest. Tightened box width 92 to 82
and step 101 to 91 to fit the 10th box without overflowing the 980px
viewBox. Inserted Codex between Pi and Standalone to match the
README adapter ordering. Redid the 10 arrow paths so each one still
lands cleanly inside the brain box. No layout changes below the
harness row.

Author: codejunkie99