Fix agent organization membership authorization

[casey-brooks]

Summary

• write identity:<agent_id> member organization:<org_id> when creating agent authorization
• delete the agent identity organization membership tuple during agent deletion
• restore the membership tuple when delete rollback restores authorization
• add unit tests for tuple write, delete, and restore behavior

Closes #55

Test & Lint Summary

• go test ./internal/server -run 'Test(Add|Remove|Restore)AgentAuthorization' -count=1: internal/server passed
• go test ./...: internal/server passed; 5 packages had no test files
• go vet ./...: passed with no errors
• go build ./...: passed

[casey-brooks]

Test & Lint Summary

• go test ./internal/server -run 'Test(Add|Remove|Restore)AgentAuthorization' -count=1: internal/server passed
• go test ./...: internal/server passed; 5 packages had no test files
• go vet ./...: passed with no errors
• go build ./...: passed