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>

.woodpecker/ci.yml

@@ -12,9 +12,7 @@ steps:
   - name: shellcheck
     image: koalaman/shellcheck-alpine:stable
     commands:
-      # Report warnings but don't block CI — existing code has known issues
-      # TODO: fix all warnings (#45) and remove || true
-      - find . -name "*.sh" -not -path "./.git/*" -print0 | xargs -0 -r shellcheck --severity=warning || true
+      - find . -name "*.sh" -not -path "./.git/*" -print0 | xargs -0 -r shellcheck --severity=warning
 
   - name: duplicate-detection
     image: python:3-alpine