drop stale phase-F labeling from health + main.zig
phase A-F was an internal numbering from the python spike → zig backend
migration. nothing has been "phase F" in weeks; it just reads as nonsense
in the health endpoint. /health now returns {"status":"ok"}, and the
main.zig module comment describes what ken actually does.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>