fix: use array-based session detection for robust concurrent fork handling

Replace string-based session comparison with array-based approach:

- Store before_sessions in an array instead of pipe-delimited string
- This is more robust against word-splitting issues in bash
- Skip base_session_id and pm_agent_session_id explicitly
- Compare each after-session against the before array

This approach is more reliable when multiple agents fork concurrently
because it properly compares each session ID individually rather than
relying on regex matching in a string.

Fixes #81

skills/kugetsu/scripts/kugetsu

@@ -829,8 +829,11 @@ cmd_start() {
 
     local session_file="$(issue_ref_to_filename "$issue_ref").json"
 
-    local before_sessions=$(opencode session list 2>/dev/null | grep -oP '^ses_\w+' | sort)
-    local before_set="${before_sessions//$'\n'/|}"
+    # Get list of sessions before fork to compare against after
+    declare -a before_sessions=()
+    while IFS= read -r sess; do
+        before_sessions+=("$sess")
+    done < <(opencode session list 2>/dev/null | grep -oP '^ses_\w+')
 
     echo "Forking session for '$issue_ref'..."
     
@@ -849,14 +852,31 @@ cmd_start() {
         opencode run "$message" --fork --session "$base_session_id" --dir "$worktree_path" 2>&1 &
     fi
 
-    local after_sessions=$(opencode session list 2>/dev/null | grep -oP '^ses_\w+' | sort)
+    # Wait briefly for session to be created
+    sleep 1
+
+    # Find the new session by comparing before/after lists
+    # Skip any session that existed before the fork and skip base/pm-agent
     local new_session_id=""
     while IFS= read -r sess; do
-        if [[ ! "$before_set" =~ \|${sess}\| ]] && [[ "$sess" != "$base_session_id" ]] && [[ "$sess" != "$pm_agent_session_id" ]]; then
+        # Skip base and pm-agent
+        [ "$sess" = "$base_session_id" ] && continue
+        [ "$sess" = "$pm_agent_session_id" ] && continue
+        
+        # Check if this session existed before
+        local existed_before=false
+        for before_sess in "${before_sessions[@]}"; do
+            if [ "$sess" = "$before_sess" ]; then
+                existed_before=true
+                break
+            fi
+        done
+        
+        if [ "$existed_before" = false ]; then
             new_session_id="$sess"
             break
         fi
-    done <<< "$after_sessions"
+    done < <(opencode session list 2>/dev/null | grep -oP '^ses_\w+')
 
     if [ -z "$new_session_id" ]; then
         echo "Error: Could not find newly created session" >&2