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>

lib/env.sh

@@ -82,7 +82,8 @@ matrix_send() {
   [ -z "${MATRIX_TOKEN:-}" ] && return 0
   local prefix="$1" msg="$2" thread_id="${3:-}" ctx_tag="${4:-}"
   local room_encoded="${MATRIX_ROOM_ID//!/%21}"
-  local txn="$(date +%s%N)$$"
+  local txn
+  txn="$(date +%s%N)$$"
   local body
   if [ -n "$thread_id" ]; then
     body=$(jq -nc --arg m "[${prefix}] ${msg}" --arg t "$thread_id" \