wave chat: Phase 3 — cascade control

[nextlevelshit]

Parent: #113
Depends on: Phase 2 (step manipulation)

Problem

After rewriting or reverting a step, downstream steps may consume stale artifacts. Users need visibility into what's stale and control over how to propagate changes.

Design

Stale artifact detection

Walk the pipeline DAG forward from a modified step. For each downstream step, check if its injected artifacts have been modified (mtime comparison against step completion time).

// Added to StepController interface
GetStaleDownstream(runID, stepID string) ([]StaleStep, error)
CascadeFrom(ctx context.Context, runID, stepID string, targets []string) error

type StaleStep struct {
    StepID       string
    Reasons      []string // Why it's stale (e.g., "consumes artifact X from modified step Y")
    AffectedArtifacts []string
}

Reuse existing infrastructure

• StaleArtifactDetector from validation.go — already detects mtime-based staleness
• ResumeManager.createResumeSubpipeline() — creates subpipeline from step forward
• ResumeManager.executeResumedPipeline() — executes subpipeline
• DAG topology from DAGValidator.TopologicalSort()

Cascade modes

After detecting stale steps, present the user with options:

1. Cascade: Re-run all stale downstream steps automatically
2. Selective: Let user pick which steps to re-run (interactive prompt)
3. Stop: Leave stale artifacts as-is (user acknowledges staleness)

Implementation approach

1. Generalize StaleArtifactDetector to work with any pipeline (currently prototype-only)
2. Add GetStaleDownstream() that walks inject_artifacts references forward
3. Add CascadeFrom() that creates a subpipeline of selected targets and executes
4. Add stale markers to state store (tag-based: stale:<step-id>)
5. Update wave chat workspace CLAUDE.md to include stale step warnings

Files to create/modify

File	Action
internal/pipeline/cascade.go	Create — Cascade detection and execution
internal/pipeline/cascade_test.go	Create — Tests
internal/pipeline/validation.go	Modify — Generalize StaleArtifactDetector
internal/pipeline/stepcontroller.go	Modify — Add cascade methods
internal/pipeline/chatworkspace.go	Modify — Stale warnings in CLAUDE.md

Acceptance criteria

• GetStaleDownstream() correctly identifies all steps consuming artifacts from modified step
• CascadeFrom() re-runs selected steps in topological order
• Stale detection works for any pipeline (not just prototype)
• Chat workspace shows stale warnings
• All tests pass with -race