Remove JFS references + secrets engine injection

[saucow]

What I did

• Removed JFS socket API references and added se:// URI-based secret injection with Secrets Engine.
• Builds upon cmd/secret: use new store #217 and Remove jfs #220

Changes

Secret Injection

• Containers: Inject se://docker/mcp/generic/{secretName} URIs, resolved by Docker Desktop at runtime
• Remote MCPs: Query Secrets Engine for actual values (HTTP headers require plaintext)
• Gateway: No longer holds secret values in memory

Deleted:

• pkg/desktop/secrets.go - JFS client
• pkg/docker/secrets.go - jcat container mechanism
• cmd/docker-mcp/commands/policy.go - Policy commands

Removed:

• Secret backup/restore from backup/dump.go and backup/restore.go
• Policy fields from Backup struct
• Secret reading from configuration loading (no more jcat containers)

Updated Components

• pkg/gateway/clientpool.go - Build se:// URIs from secret names
• pkg/gateway/configuration.go - Return empty secrets map
• pkg/mcp/remote.go - Query Secrets Engine for remote MCP headers
• cmd/docker-mcp/server/secret_config.go - Use Secrets Engine for server ls

Remaining:

• OAuth related