fix(kugetsu): init creates base session in ~/.kugetsu-worktrees and adds context to forked sessions

1. Init: cd to ~/.kugetsu-worktrees before creating base session
   This keeps all worktrees inside a predictable directory structure
   and avoids external_directory permission issues

2. Fork context: Add kugetsu_get_fork_context() that provides:
   - Important working rules (stop on error, don't pivot)
   - Repository configuration from repos.json
   - Environment file location info

3. Fork message: Prepend context to user message when forking session

skills/kugetsu/scripts/kugetsu

@@ -394,6 +394,41 @@ kugetsu_get_pm_context() {
     fi
 }
 
+kugetsu_get_fork_context() {
+    local issue_ref="$1"
+    local context=""
+    
+    context="## IMPORTANT WORKING RULES
+
+1. You are working on issue: $issue_ref
+2. If you encounter ANY error, blocker, or cannot complete the task:
+   - STOP immediately
+   - Log what happened and why you cannot proceed
+   - Do NOT switch to other work or try alternative approaches
+3. Do NOT work on other issues or PRs unless explicitly asked
+4. Environment variables are available in ~/.kugetsu/env/
+
+"
+    
+    if [ -f "$REPOS_CONFIG" ]; then
+        context="${context}
+## REPOSITORIES CONFIG
+$(cat "$REPOS_CONFIG")
+
+"
+    fi
+    
+    if [ -f "$ENV_DIR/default.env" ]; then
+        context="${context}
+## ENVIRONMENT (available at ~/.kugetsu/env/)
+Environment file exists at: $ENV_DIR/default.env
+Source it with: source ~/.kugetsu/env/default.env
+"
+    fi
+    
+    echo "$context"
+}
+
 kugetsu_add_notification() {
     local type="$1"
     local message="$2"
@@ -1107,12 +1142,19 @@ EOF
         exit 1
     fi
 
+    local init_worktree_dir="$HOME/.kugetsu-worktrees"
+    mkdir -p "$init_worktree_dir"
+    cd "$init_worktree_dir"
+    echo "Initialized kugetsu worktrees directory: $init_worktree_dir"
+    echo "Base session will be created in this directory."
+    echo ""
+
     local cwd_files=$(ls -A "$PWD" 2>/dev/null | wc -l)
     local cwd_git=$(git rev-parse --is-inside-work-tree 2>/dev/null || echo "false")
     if [ "$cwd_files" -gt 0 ] || [ "$cwd_git" = "true" ]; then
-        echo "Warning: Current directory is not empty: $PWD" >&2
+        echo "Warning: Worktrees directory is not empty: $PWD" >&2
         echo "This may cause project context to contaminate the base session." >&2
-        echo "Consider running kugetsu init from an empty directory." >&2
+        echo "Consider running kugetsu destroy --base -y and reinitializing." >&2
         echo "" >&2
         echo "Files in current directory: $cwd_files" >&2
         if [ "$cwd_git" = "true" ]; then
@@ -1257,13 +1299,19 @@ cmd_start() {
     local opencode_db="${OPENCODE_DB:-$HOME/.local/share/opencode/opencode.db}"
     
     > "$fork_log"
-     
+    
+    local fork_context=$(kugetsu_get_fork_context "$issue_ref")
+    local full_message="${fork_context}
+
+## YOUR TASK
+$message"
+    
     fix_session_permissions
     
     if [ "$DEBUG_MODE" = true ]; then
-        (cd "$worktree_path" && opencode run "$message" --fork --session "$base_session_id" --dir "$worktree_path" 2>&1) | tee "$fork_log" &
+        (cd "$worktree_path" && opencode run "$full_message" --fork --session "$base_session_id" --dir "$worktree_path" 2>&1) | tee "$fork_log" &
     else
-        (cd "$worktree_path" && opencode run "$message" --fork --session "$base_session_id" --dir "$worktree_path" 2>&1) >> "$fork_log" &
+        (cd "$worktree_path" && opencode run "$full_message" --fork --session "$base_session_id" --dir "$worktree_path" 2>&1) >> "$fork_log" &
     fi
 
     local fork_pid=$!
@@ -1548,6 +1596,7 @@ cmd_destroy() {
 
     if [ "$target" = "base" ]; then
         if [ "$force" = true ]; then
+            local base_session_id=$(get_base_session_id)
             rm -f "$SESSIONS_DIR/base.json"
             local pm_agent=$(get_pm_agent_session_id)
             if [ -n "$pm_agent" ] && [ "$pm_agent" != "null" ]; then
@@ -1556,6 +1605,10 @@ cmd_destroy() {
             else
                 echo '{"base": null, "pm_agent": null, "issues": {}}' > "$INDEX_FILE"
             fi
+            if [ -n "$base_session_id" ] && [ "$base_session_id" != "null" ]; then
+                echo "Deleting opencode session: $base_session_id"
+                opencode session delete "$base_session_id" 2>/dev/null || echo "Warning: Could not delete session from opencode (may already be deleted)"
+            fi
             echo "Base session destroyed"
         else
             echo "Error: destroying base session requires --base -y" >&2
@@ -1566,6 +1619,7 @@ cmd_destroy() {
 
     if [ "$target" = "pm-agent" ]; then
         if [ "$force" = true ]; then
+            local pm_session_id=$(get_pm_agent_session_id)
             rm -f "$SESSIONS_DIR/pm-agent.json"
             local base=$(get_base_session_id)
             if [ -n "$base" ] && [ "$base" != "null" ]; then
@@ -1573,6 +1627,10 @@ cmd_destroy() {
             else
                 write_index "null" "null" "{}"
             fi
+            if [ -n "$pm_session_id" ] && [ "$pm_session_id" != "null" ]; then
+                echo "Deleting opencode session: $pm_session_id"
+                opencode session delete "$pm_session_id" 2>/dev/null || echo "Warning: Could not delete session from opencode (may already be deleted)"
+            fi
             echo "PM agent session destroyed"
         else
             echo "Error: destroying pm-agent session requires --pm-agent -y" >&2