feat: sub-pipeline composition for workflow nesting

[nextlevelshit]

Context

Fabro supports sub-workflows via house shape nodes — a parent workflow can invoke a child workflow with bidirectional context flow. The child gets a clone of parent context, executes independently, and modifications merge back via diff. The parent can configure max cycles, poll intervals, and stop conditions.

Wave's meta.go supports dynamic pipeline generation but not runtime composition — you can't have a pipeline step that invokes another pipeline.

Design

Nested Pipeline Steps

steps:
  - name: plan
    persona: navigator
    
  - name: implement-and-test
    type: pipeline
    pipeline: implement-test-loop    # reference another pipeline
    depends_on: [plan]
    config:
      max_cycles: 50                  # max iterations in child
      timeout: 3600s                  # hard timeout
      stop_condition: "context.tests_pass=true"
    artifacts:
      inject: [plan]                  # pass parent artifacts to child
      extract: [implementation]        # pull child artifacts back to parent

Context Flow

1. Parent → Child: Artifacts listed in inject are copied into child's .wave/artifacts/
2. Child → Parent: Artifacts listed in extract are copied back after child completes
3. Context variables: Child's context updates merge into parent context

Use Cases

• implement-issue delegates to implement-test-loop for the implement→test→fix cycle
• speckit-flow delegates to implement for the actual coding step
• audit-dual runs audit-security and audit-dx as child pipelines

Workspace Handling

Child pipeline gets its own workspace (worktree) by default. Can share parent workspace via:

  - name: implement-and-test
    type: pipeline
    pipeline: implement-test-loop
    workspace:
      ref: parent                     # share parent's workspace

Implementation Scope

1. Add type: pipeline step type to manifest schema
2. Sub-pipeline executor — launches child pipeline within parent run
3. Artifact injection/extraction between parent and child
4. Context merging
5. Lifecycle management (timeout, stop condition, max cycles)
6. State tracking — child run is a sub-entry in parent run state

Research Sources

• Fabro sub-workflows: house shape with stack.child_workflow, manager.max_cycles, manager.stop_condition
• Fabro context flow: bidirectional context clone + diff merge

[nextlevelshit]

Implemented and merged as part of epic #589. See #609 for session diary.