fix: shellcheck TODO has no enforcement — || true may never be removed (#71)

- Fix SC2164: add || exit 1 to bare cd in update-prompt.sh
- Fix SC2155: separate declare and assign in env.sh, supervisor-poll.sh, dev-agent.sh
- Fix SC2034: inline suppression for vars used by sourced helpers
- Remove unused `mergeable` declaration, rename unused loop var to `_w`
- Remove || true from shellcheck CI step — failures are now blocking

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

supervisor/update-prompt.sh

@@ -39,7 +39,7 @@ else
   echo "$LESSON" >> "$TARGET_FILE"
 fi
 
-cd "$FACTORY_ROOT"
+cd "$FACTORY_ROOT" || exit 1
 git add "supervisor/$1" 2>/dev/null || git add "$TARGET_FILE"
 git commit -m "supervisor: learned — $(echo "$LESSON" | head -1 | sed 's/^#* *//')" --no-verify 2>/dev/null
 git push origin main 2>/dev/null