feat(workspace): deferred workspace creation — reference prior step outputs in workspace config

[nextlevelshit]

Problem

Workspace configuration is resolved at pipeline parse time, before any steps execute. This means workspace fields like branch: can only use static values or pipeline-level template variables ({{ pipeline_id }}), not outputs from prior steps.

This forces patterns like:

1. Create a throwaway worktree on {{ pipeline_id }}
2. Inside the step prompt, git fetch && git checkout the real branch
3. The worktree creation was wasted work

This matters most for cross-pipeline workflows like ops-pr-rework, where the target branch name comes from a prior step's output (the PR's headRefName).

Proposed Solution

Support step output references in workspace config:

steps:
  - id: fetch-review
    # ... produces artifact with head_branch field
    
  - id: apply-fixes
    dependencies: [fetch-review]
    workspace:
      type: worktree
      branch: "{{ steps.fetch-review.artifacts.review-findings.head_branch }}"

The workspace for apply-fixes is created after fetch-review completes, using the resolved value from its output artifact.

Implementation

Deferred resolution in executor

In internal/pipeline/executor.go, the workspace creation path currently runs before step execution. Change to:

1. At step start, check if workspace config contains {{ steps.* }} references
2. If yes, resolve the references from prior step artifacts (already available in .wave/artifacts/)
3. Then create the workspace with the resolved config

This is a narrow change — only the workspace creation path needs to defer, not the full config resolution.

Template syntax

Reuse the existing template variable system. Add a steps namespace:

• {{ steps.<step-id>.artifacts.<artifact-name>.<json-path> }} — read a field from a prior step's JSON artifact
• {{ steps.<step-id>.output.<field> }} — read from step outcome metadata

Fallback

If the referenced step hasn't completed or the artifact doesn't exist, fail with a clear error rather than creating a broken workspace.

Acceptance Criteria

• Workspace branch: field can reference prior step artifact values
• Workspace creation is deferred until references are resolved
• Clear error if referenced step/artifact doesn't exist
• Existing static workspace configs unaffected
• ops-pr-rework pipeline updated to use deferred branch resolution