fix(docker): B5 tighten bash-guard denial message to 2 lines

Smoke run 3 showed the bash-guard hook emitting 8+ lines on every
blocked shell-git op — enumerating every alternative MCP verb across
roboco-flow / roboco-do / roboco-git-readonly. That's repeated token
spend on every refused retry; the LLM doesn't need the full alt-list
inline, it has the role prompt + the MCP tool schema for that.

Trimmed to 2 lines: denial reason + a one-line pointer to the role's
State→Verb table. Test asserts <= 3 echo lines in any denial block.

Spec ref: docs/superpowers/specs/2026-05-12-post-smoke-3-fixes-design.md
section B5.

tests/integration/test_bash_guard_message.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+# Smoke: bash-guard denial message is <= 3 lines (was 8+).
+# Smoke run 3 (2026-05-12) showed agents getting an 8-line refusal on
+# every blocked shell-git op. Tighter messages save tokens on retries.
+set -e
+
+cd "$(dirname "$0")/../.."
+
+HOOK=docker/scripts/bash-guard-hook.sh
+
+# Count lines in the longest heredoc denial block (cat <<'EOF' ... EOF style).
+# The first git-network denial used a heredoc; awk counts echo-or-content
+# lines inside it.
+HEREDOC_LINES=$(awk '
+  /cat <<'"'"'EOF'"'"'/ { in_block = 1; count = 0; next }
+  in_block && /^EOF$/ { print count; in_block = 0; count = 0; next }
+  in_block { count++ }
+  END { if (in_block && count > 0) print count }
+' "$HOOK" | sort -n | tail -1)
+
+# Also count contiguous echo-based denial blocks (other denial paths).
+ECHO_LINES=$(awk '
+  /Denied/ { in_block = 1; count = 0 }
+  in_block && /^[[:space:]]*echo/ { count++ }
+  in_block && /^[[:space:]]*$/ && count > 0 { print count; in_block = 0; count = 0 }
+  END { if (in_block && count > 0) print count }
+' "$HOOK" | sort -n | tail -1)
+
+MAX_LINES=0
+[[ -n "$HEREDOC_LINES" && "$HEREDOC_LINES" -gt "$MAX_LINES" ]] && MAX_LINES=$HEREDOC_LINES
+[[ -n "$ECHO_LINES" && "$ECHO_LINES" -gt "$MAX_LINES" ]] && MAX_LINES=$ECHO_LINES
+
+if [ "$MAX_LINES" -eq 0 ]; then
+    echo "FAIL: could not find any denial message block in $HOOK"
+    exit 1
+fi
+
+if [ "$MAX_LINES" -gt 3 ]; then
+    echo "FAIL: bash-guard denial message has $MAX_LINES lines (max 3)"
+    exit 1
+fi
+
+echo "PASS: denial message has $MAX_LINES lines"