feat(workflows): add pipeline step type for in-process command execution

[sfo2001]

Motivation

Workflow steps currently execute via /bin/sh, so using lobster's stdlib commands (map, sort, jq-filter, each) from a workflow requires spawning a separate lobster process. The pipeline step type runs commands in-process through the existing registry, avoiding the shell round-trip and making stdlib commands first-class citizens in workflow definitions.

Summary

• Workflow steps can now use pipeline: "command | chain" instead of command: "shell string"
• Pipeline steps run in-process via the command registry (no shell spawn), enabling use of stdlib commands like map, sort, each directly in workflow YAML
• Exactly one of command or pipeline is required per step; validation rejects both or neither
• cwd is not supported for pipeline steps (raises clear error)
• Passes registry through RunContext from CLI to workflow runner

Changes

Category	Files
Workflow runner	src/workflows/file.ts (+runPipelineStep, stdinToStream, validation)
CLI	src/cli.ts (pass registry into workflow run context)
Registry	src/commands/registry.ts (export Registry interface)
Tests	test/workflow_pipeline_step.test.ts (268 lines)

+366 / -9 lines across 4 files.

Test plan

• pnpm build passes
• pnpm lint passes
• node --test dist/test/workflow_pipeline_step.test.js
• Existing workflow tests still pass (backward-compatible, command steps unchanged)

Use of AI

• my idea, code and test generated with Claude Code