feat: add named workspaces and workspace listing

Support WORKSPACE=<name> flag for friendly workspace names that
auto-resume if they exist or create a new named workspace otherwise.
Add ./shannon workspaces command to list all workspaces with status,
duration, and cost.

src/temporal/shared.ts

@@ -9,6 +9,7 @@ export interface PipelineInput {
   outputPath?: string;
   pipelineTestingMode?: boolean;
   workflowId?: string; // Added by client, used for audit correlation
+  sessionId?: string; // Workspace directory name (distinct from workflowId for named workspaces)
   resumeFromWorkspace?: string; // Workspace name to resume from
   terminatedWorkflows?: string[]; // Workflows terminated during resume
 }