Interpolating inputs straight into the JSON string let a quote in image or
tag break the payload. Also replaces GNU-only head -n -1 so the action runs
on BSD/macOS runners.
Captures design intent, gotchas, and references that aren't obvious
from code alone — single-worker queue rationale, defense-in-depth
security model, the not-yet-wired selfupdate package, and pointers
to the design spec and implementation plan in docs/superpowers/.