docs: fix broken composition link in langchain integration (unblocks strict mkdocs build)

The langchain integration page linked ../composition.md#langchain, but the
file lives at concepts/composition.md (and has no langchain anchor). This
broke 'mkdocs build --strict' — the Docs workflow had been failing since
2026-06-09, so the hosted site never redeployed. Point it at the real
composition runbook.

Author: valani9

docs/integrations/langchain.md

@@ -290,7 +290,7 @@ result = diagnosed_invoke(chain, {"question": "..."}, goal="...")
 
 ## See also
 
-- LangChain-specific [composition graphs](../composition.md#langchain)
+- The [composition runbook](../concepts/composition.md) — how patterns chain together
 - Adapter source: `_adapters/lib/langchain_core_adapter.py`
 - LangGraph example: `examples/cookbook/09_bottleneck_orchestrator.py`
 - Live LangChain demo: `examples/patterns/01_lewin.py`