refactor(langgraph): adapter reflects CompiledStep.executionPlan on node metadata

[SingleSourceStudios]

Part of #75 (LangGraph adapter half). Companion to the executor half.

Summary

#65 added CompiledStep.executionPlan (ExecutionPlan | null). The LangGraph adapter built nodes without it, so the serialized state graph carried no fan-out/fan-in. This reflects the compiled plan onto StateGraphNode.metadata.executionPlan.

• Read from compiledStep.executionPlan (the adapter already holds the CompiledStep in buildGraphDefinition), gated by includeMetadata and omitted when null, mirroring the existing retryPolicy reflection. A LangGraph builder can construct parallel/conditional edges from this without reaching back into the un-compiled spec.
• Consume-and-reflect only; no concurrent scheduler.

Tests

• Loop-closing test (src/__tests__/execution-plan-source.test.ts) wraps compileWorkflow to attach an executionPlan to the compiled step while the source spec declares no parallel structure, proving the adapter reflects the compiled artifact and not the spec. Verified RED before the fix, GREEN after.

Reviewer-relevant caveats (please read before merge)

The adapters/langgraph package is outside all root gate commands: it is not in the root workspaces (packages/* only), is excluded from biome (!**/adapters), is not referenced by tsconfig.build.json, and is not a vitest project (root vitest projects: ["packages/*"]). Root npm test / typecheck / lint therefore do not exercise it and remain green.

On main, before this change, the adapter package already fails its own suite (4 failing tests) and does not typecheck (~8 tsc --noEmit errors) due to stale drift against the current core API (console/lib config, ParseResult.data, DagResult.levels/order) — all unrelated to executionPlan. This change does not introduce or fix any of that; it is purely additive (one metadata field + one passing test), verified by running the adapter suite directly (my test passes; the 4 pre-existing failures persist unchanged).

Recommendation: merge the executor half (#85) unconditionally. For this PR, either merge as-is and track a follow-up to repair the adapter's API drift and bring it under a gate, or fold this field into that repair so the adapter re-enters CI green in one move. I did not touch root config or repair the drift, as both are out of scope for #75.

🤖 Generated with Claude Code

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Summary by cubic

Reflects the compiled parallel execution plan onto LangGraph node metadata so consumers can build fan-out/fan-in edges without reading the source spec. Part of #75; no scheduler changes.

• Refactors
  • Read from CompiledStep.executionPlan and write to StateGraphNode.metadata.executionPlan when includeMetadata is true; omitted when null.
  • Updated types to include ExecutionPlan from @logic-md/core and added a test verifying the adapter uses the compiled artifact, not the spec.

^{Written for commit 0560a24. Summary will update on new commits. Review in cubic}

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📥 Commits

Reviewing files that changed from the base of the PR and between 6eddce5 and 0560a24.

📒 Files selected for processing (3)

• adapters/langgraph/src/__tests__/execution-plan-source.test.ts
• adapters/langgraph/src/adapter.ts
• adapters/langgraph/src/types.ts

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• Create PR with unit tests
• Commit unit tests in branch refactor/consume-execution-plan-adapter

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