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>

dev/dev-agent.sh

@@ -28,7 +28,9 @@ git -C "$FACTORY_ROOT" pull --ff-only origin main 2>/dev/null || true
 
 # --- Config ---
 ISSUE="${1:?Usage: dev-agent.sh <issue-number>}"
+# shellcheck disable=SC2034
 REPO="${CODEBERG_REPO}"
+# shellcheck disable=SC2034
 REPO_ROOT="${PROJECT_REPO_ROOT}"
 
 API="${CODEBERG_API}"
@@ -143,7 +145,8 @@ post_refusal_comment() {
     log "skipping duplicate refusal comment: ${title}"
     return 0
   fi
-  local comment="${emoji} **Dev-agent: ${title}**
+  local comment
+  comment="${emoji} **Dev-agent: ${title}**
 
 ${body}