Why: a ContractRequest without a workspace (the conformance minimal
case) resolved WorkDir to the process CWD and wrote .agentx transcripts
into the repo, dirtying the tree in CI and failing the build step's
worktree check.
Evidence: GOWORK=off go vet + go test -count=1 ./... green;
TestContractEmptyWorkspaceKeepsCWDClean pins the fixture bytes;
the adapters/native tree no longer drifts after a suite run.
Impact: requests with an explicit workspace are unchanged; empty
workspace now lands in a disposable temp dir.
Refs: Agent-System-Dev#242 CI failure (run 1144)