Expand description
Heddle CLI exit code taxonomy.
Agents that retry on transient failures need codified exit codes so they
can distinguish “safe to retry” from “permanent failure” without parsing
stderr. The taxonomy follows BSD sysexits.h so the codes mean the same
thing to humans, init systems, and shell scripts that already understand
them.
0 is success; 2 is reserved for set -e / panic / unhandled error and
is never emitted intentionally — we let it surface naturally.