docs(adr): add ADR-0006 for workspace container architecture

Define the standard architecture for agentic sessions:
- Separate agent and workspace containers with kubectl exec
- Cascading configuration: Platform → Project → Session
- workspacePodTemplate for full Kubernetes pod customization
- MCP-based tool replacement (disallow Bash, provide mcp__workspace__exec)
- RBAC setup for pods/exec permission

Co-Authored-By: Claude <noreply@anthropic.com>
Assisted-by: Claude Code (Opus 4.5)

Author: cgwalters