feat: add workflow extraction Pass 2 (transcript to structured workflow)

[abrichr]

Summary

• Implements Pass 2 of the 4-pass workflow pipeline: extracts structured Workflow objects from EpisodeTranscript (Pass 1 output)
• Uses a single VLM call per transcript to identify and merge related actions into high-level WorkflowStep entries (e.g., click + type -> "Enter header 'Year' in cell A1")
• Includes graceful fallback: if VLM parsing fails, maps each transcript entry 1:1 to a WorkflowStep so downstream passes (Pass 3 matching) still work

Details

• New file: openadapt_evals/workflow/pipeline/extract.py with extract_workflow() public API
• Updated pipeline/__init__.py to export extract_workflow and fix docstring listing all 4 passes
• Follows the same VLM client pattern as transcript.py (uses vlm_call + extract_json from openadapt_evals.vlm)
• VLM prompt asks for structured JSON with think/action/expect per step, action classification, parameters, and source entry provenance
• Corrective actions are filtered out in fallback mode; the VLM prompt instructs skipping them in normal mode

Test plan

• Verify ruff check and ruff format pass (confirmed locally)
• Verify from openadapt_evals.workflow.pipeline import extract_workflow imports cleanly
• Integration test with a real EpisodeTranscript + VLM API key
• Verify output Workflow is accepted by Pass 3 (match_workflow_to_canonical)

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