fix: replace O(n^2) escape_for_json with parameter substitution

The character-by-character loop using ${input:$i:1} was O(n^2) in
bash due to substring copy overhead. On Windows Git Bash this took
60+ seconds, freezing terminal input even with async hooks.

Replaced with bash parameter substitution (${s//old/new}) which runs
each pattern as a single C-level pass. 7x faster on macOS, expected
to be dramatically faster on Windows Git Bash where the original
caused the worst hangs.

Relates to #404, #413

hooks/session-start.sh

@@ -17,23 +17,17 @@ fi
 # Read using-superpowers content
 using_superpowers_content=$(cat "${PLUGIN_ROOT}/skills/using-superpowers/SKILL.md" 2>&1 || echo "Error reading using-superpowers skill")
 
-# Escape outputs for JSON using pure bash
+# Escape string for JSON embedding using bash parameter substitution.
+# Each ${s//old/new} is a single C-level pass - orders of magnitude
+# faster than the character-by-character loop this replaces.
 escape_for_json() {
-    local input="$1"
-    local output=""
-    local i char
-    for (( i=0; i<${#input}; i++ )); do
-        char="${input:$i:1}"
-        case "$char" in
-            $'\\') output+='\\' ;;
-            '"') output+='\"' ;;
-            $'\n') output+='\n' ;;
-            $'\r') output+='\r' ;;
-            $'\t') output+='\t' ;;
-            *) output+="$char" ;;
-        esac
-    done
-    printf '%s' "$output"
+    local s="$1"
+    s="${s//\\/\\\\}"
+    s="${s//\"/\\\"}"
+    s="${s//$'\n'/\\n}"
+    s="${s//$'\r'/\\r}"
+    s="${s//$'\t'/\\t}"
+    printf '%s' "$s"
 }
 
 using_superpowers_escaped=$(escape_for_json "$using_superpowers_content")