feat: implement cmd management for targets, LUNs, and TPGTs (fixes #36)

- Fix target delete URL path mismatch (/targets/ -> /target/)
- Implement target create/delete server handlers with proper validation
- Add DeleteTarget method with force flag and mutex locking to SCSITargetService
- Implement full LU management: create/list/delete through CLI, client, and server
- Add TPGT list command to show target portal group tags
- Add unit tests for target/LU router handlers and SCSI service

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/api/client/lu_delete.go

@@ -0,0 +1,13 @@
+package client
+
+import (
+	"github.com/gostor/gotgt/pkg/api"
+	"golang.org/x/net/context"
+)
+
+// LuRemove removes a LU from a target in the SCSI Target.
+func (cli *Client) LuRemove(ctx context.Context, options api.LuRemoveOptions) error {
+	resp, err := cli.post(ctx, "/lu/delete", nil, options, nil)
+	ensureReaderClosed(resp)
+	return err
+}