docs: focus README and refresh brand art

[Ghost-Frame]

Summary

• reduce the public README from 6,184 words to 1,181 words and route detailed reference material to the maintained wiki
• separate hosted desktop downloads from the public FrameShift Desktop source repository
• replace the legacy banner with the current editorial art direction
• add a square FrameShift social avatar based on the interlocking-frame brand mark

Verification

• Agent Forge verification: 9 of 9 checks passed
• canonical wiki validation passed
• all relative README links resolve
• banner contract: 1536 x 768 RGB PNG
• social avatar contract: 1024 x 1024 RGB PNG
• exact and perceptual duplicate checks passed
• pre-push formatting and RustSec audit policy passed