feat: auto-instrument LangGraph nodes and routing decisions

[DudeFr0mMars]

Adds chronicle.instrument(graph): one call auto-instruments every node and every add_conditional_edges routing function on a LangGraph StateGraph, before or after .compile().

Works on the builder's public attributes (nodes, branches), not langgraph's private compiled Pregel internals — more stable across langgraph versions.
Also works after .compile(): a CompiledStateGraph keeps a live reference to its builder (compiled.builder), and node callables are mutated in place, so late instrumentation still reroutes already-compiled execution.
Routing functions become kind="router" boundaries, so which branch was taken is recorded and replays deterministically from a fixture — not just each node's input/output. session.py gained a small kind="router" branch in the record/replay round-trip so a router's plain string/list return value survives stub replay faithfully (it isn't a dict, so it can't just be handed back as raw_response).
Handles async correctly, which needed two fixes verified by test, not assumed: (1) langgraph gives a sync-only node an auto-generated async executor shim that closes over the original function — rewiring only the sync side would silently skip recording under ainvoke/astream, so the shim is rebuilt around the instrumented function instead; (2) that shim runs in a thread pool, and ContextVars (how Chronicle scopes its session) don't cross threads automatically, so the caller's context is copied into the executor thread — the same fix langgraph's own private run_in_executor helper applies, for the same reason.
Idempotent: calling instrument() twice on the same graph wraps each node/router once.
Also adds examples/langgraph_demo/routing_demo.py (record → replay a 3-node graph with a conditional edge; replay reproduces the same completion with no node/router code executing).

Related issue
Closes #22

Checklist

Checklist

• Tests added or updated, and pytest -v passes locally
• ruff check . is clean
• Docs / CHANGELOG updated if user-facing
• If the Envelope schema changed: bumped at least a minor version and included a fixture migration path (see RELEASING.md)
• Commits are signed off for the DCO (git commit -s)