docs(readme): tighten Why AMD + add For Deaf-led teams section

fix: 4 BLOCKERs + 6 IMPORTANTs from deep-check audit

feat(submission): generate 1280x640 cover image for HF Space + lablab

fix(license): switch from Apache 2.0 to MIT to satisfy hackathon rule

fix: shorten README short_description to <60 chars (HF Space requirement)

chore: drop Build-in-Public extra challenge

feat: SignBridge initial scaffold + tests