docs: replace GNU-only head -n -1 in fan-out plan with bash expansion

head -n -1 fails on BSD/macOS, making the plan's local verification steps
unrunnable. Bash parameter expansion is portable and drops two subprocesses
per endpoint.

Claude-Session: https://claude.ai/code/session_01S3aqJ4tvaPezQhsGNCybut

docs/superpowers/plans/2026-08-04-multi-server-fanout.md

@@ -285,8 +285,11 @@ runs:
             continue
           fi
 
-          body=$(printf '%s' "$response" | head -n -1)
-          code=$(printf '%s' "$response" | tail -n 1)
+          # Split on the last newline: curl's -w appended the status code
+          # there. Pure bash — `head -n -1` is GNU-only and fails on
+          # BSD/macOS, so the fan-out could not be tested locally.
+          code="${response##*$'\n'}"
+          body="${response%$'\n'*}"
           echo "HTTP $code"
           printf '%s' "$body" | jq . || printf '%s\n' "$body"
 
@@ -306,11 +309,12 @@ runs:
         echo "all $attempted endpoint(s) updated"
 ```
 
-Three changes beyond the loop itself, each load-bearing:
+Four changes beyond the loop itself, each load-bearing:
 
 1. **All inputs moved into `env:`.** A multi-line `${{ inputs.endpoint }}` interpolated directly into the script body would break it syntactically. Moving the others too keeps one consistent style in a short script, and stops a value containing shell metacharacters from being executed.
 2. **`jq -nc` builds the payload** instead of hand-interpolating into a JSON string. A tag containing a quote previously produced malformed JSON and a confusing 400.
 3. **`set -e` dropped**, `-uo pipefail` kept. This is the whole point of the task — see the comment in the script.
+4. **`head -n -1` replaced by bash parameter expansion.** The existing action used `head -n -1`, which is GNU-only — it fails on BSD/macOS with `illegal line count`. That made the fan-out logic impossible to test on a developer machine, so the portability fix is what makes Steps 2-4 runnable at all. It also drops two subprocesses per endpoint.
 
 - [ ] **Step 2: Verify the loop continues past a failure**
 
@@ -364,8 +368,8 @@ while IFS= read -r endpoint; do
     continue
   fi
 
-  body=$(printf '%s' "$response" | head -n -1)
-  code=$(printf '%s' "$response" | tail -n 1)
+  code="${response##*$'\n'}"
+  body="${response%$'\n'*}"
   echo "HTTP $code"
   printf '%s' "$body" | jq . || printf '%s\n' "$body"