Support Docker Sandboxes v3 kits for Chainloop Trace #3598

The Chainloop kit for Docker Sandboxes uses the v2 kit format. Docker Sandboxes moved to v3 kits, and v2 and v3 kits cannot be used together.

Replace the v2 kit with a v3 Chainloop Trace kit that users add to Docker's Claude Code kit. It keeps the current behavior: it records the session and refuses to start without credentials or Trace setup.

• v3 kit with the Chainloop CLI built in
• Release workflow tests, signs, publishes and attests the kit
• README and sbxenv.yaml updated