refactor(session): make cmd_continue idempotent with ensure_* functions

- Add ensure_worktree() - creates worktree if missing, returns status
- Add ensure_session() - creates session if missing, handles inconsistent states
- Add fork_agent() - extracted agent forking logic
- Refactor cmd_continue() to use ensure_* functions (idempotent)
- Make cmd_start() a thin wrapper calling cmd_continue()
- Simplify daemon to always call cmd_continue (no existence check)

This makes cmd_continue truly idempotent - it will:
- Continue existing session if it exists
- Create session and worktree if they don't exist
- Clean and recreate if state is inconsistent

Closes #168

skills/kugetsu/scripts/kugetsu-session.sh

@@ -386,6 +386,12 @@ ensure_session() {
         return 0
     fi
     
+    if $worktree_exists && ! $session_exists; then
+        log "warn" "ensure_session" "Worktree exists but session is missing. Removing worktree to recreate both..."
+        remove_worktree_for_issue "$issue_ref"
+        worktree_exists=false
+    fi
+    
     if ! $worktree_exists && $session_exists; then
         log "warn" "ensure_session" "Session exists but worktree is missing. Removing stale session..."
         rm -f "$session_path"