refactor: modularize kugetsu shell script

Split the monolithic kugetsu script into modular components:

Modules created:
- kugetsu-config.sh - Config/env loading and global variables
- kugetsu-index.sh - Index.json read/write via JSON
- kugetsu-worktree.sh - Git worktree operations
- kugetsu-log.sh - Structured logging and notifications
- kugetsu-session.sh - Session create/fork/destroy logic
- kugetsu-queue-daemon.sh - Queue daemon subprocess

Main script (kugetsu) is now a thin dispatcher that sources all modules.

Acceptance Criteria:
- All existing commands work exactly as before
- Main script sources modules
- Each module is independently testable

Fixes #116

skills/kugetsu/scripts/kugetsu-config.sh

@@ -0,0 +1,57 @@
+#!/bin/bash
+set -euo pipefail
+
+KUGETSU_DIR="${KUGETSU_DIR:-$HOME/.kugetsu}"
+SESSIONS_DIR="$KUGETSU_DIR/sessions"
+WORKTREES_DIR="$KUGETSU_DIR/worktrees"
+REPOS_CONFIG="$KUGETSU_DIR/repos.json"
+INDEX_FILE="$KUGETSU_DIR/index.json"
+NOTIFICATIONS_FILE="$KUGETSU_DIR/notifications.json"
+LOGS_DIR="$KUGETSU_DIR/logs"
+ENV_DIR="${ENV_DIR:-$KUGETSU_DIR/env}"
+VERBOSITY_DIR="$KUGETSU_DIR/verbosity"
+
+MAX_CONCURRENT_AGENTS="${MAX_CONCURRENT_AGENTS:-3}"
+KUGETSU_VERBOSITY="${KUGETSU_VERBOSITY:-default}"
+CONTEXT_DIR="${CONTEXT_DIR:-$KUGETSU_DIR/context}"
+ENABLE_CONTEXT_DUMP="${ENABLE_CONTEXT_DUMP:-true}"
+WORKTREE_CHECK_PR_STATUS="${WORKTREE_CHECK_PR_STATUS:-true}"
+
+QUEUE_DIR="${QUEUE_DIR:-$KUGETSU_DIR/queue}"
+QUEUE_ITEMS_DIR="${QUEUE_ITEMS_DIR:-$QUEUE_DIR/items}"
+QUEUE_DAEMON_PID_FILE="${QUEUE_DAEMON_PID_FILE:-$QUEUE_DIR/daemon.pid}"
+QUEUE_DAEMON_LOCK_FILE="${QUEUE_DAEMON_LOCK_FILE:-$QUEUE_DIR/daemon.lock}"
+QUEUE_DAEMON_LOG_FILE="${QUEUE_DAEMON_LOG_FILE:-$QUEUE_DIR/daemon.log}"
+QUEUE_DAEMON_INTERVAL_MINUTES="${QUEUE_DAEMON_INTERVAL_MINUTES:-5}"
+QUEUE_CLEANUP_AGE_DAYS="${QUEUE_CLEANUP_AGE_DAYS:-7}"
+TASK_TIMEOUT_HOURS="${TASK_TIMEOUT_HOURS:-1}"
+
+# Load user config overrides (~/.kugetsu/config)
+if [ -f "$KUGETSU_DIR/config" ]; then
+    source "$KUGETSU_DIR/config"
+fi
+
+mask_sensitive_vars() {
+    local line="$1"
+    for var in GITEA_TOKEN GITHUB_TOKEN GITLAB_TOKEN API_KEY PASSWORD TOKEN SECRET; do
+        if [[ "$line" =~ $var ]]; then
+            line=$(echo "$line" | sed -E "s/=.*/=***MASKED***/")
+        fi
+    done
+    echo "$line"
+}
+
+load_agent_env() {
+    local agent_type="${1:-base}"
+    local env_file="$ENV_DIR/${agent_type}.env"
+    
+    if [ -f "$env_file" ]; then
+        set -a
+        source "$env_file"
+        set +a
+    elif [ -f "$ENV_DIR/default.env" ]; then
+        set -a
+        source "$ENV_DIR/default.env"
+        set +a
+    fi
+}