fix: update langchain docs for passthrough guard pattern

- Enforcement mode table: monitor/log set context vars, not dict keys
- Add CapiscioRequestContext read-back example (verified, claims, warnings)
- Clarify guard is a pure passthrough — returns input unchanged

Author: beonde

docs/how-to/integrations/langchain.md

@@ -63,8 +63,12 @@ guard = CapiscioGuard(mode="log")      # Log only — good for development
 | Mode | On missing/invalid badge |
 |------|--------------------------|
 | `block` | Raises `CapiscioTrustError` |
-| `monitor` | Logs warning, adds `capiscio_warnings` to input, continues |
-| `log` | Logs info, adds `capiscio_warnings` to input, continues |
+| `monitor` | Logs warning, sets `verified=False` and `warnings` in `CapiscioRequestContext`, continues |
+| `log` | Logs info, sets `verified=False` and `warnings` in `CapiscioRequestContext`, continues |
+
+In all modes the guard is a **pure passthrough** — it returns the input unchanged.
+Verification metadata is stored in the `CapiscioRequestContext` context variable,
+not in the output.
 
 ---
 
@@ -208,6 +212,17 @@ set_capiscio_context(CapiscioRequestContext(
 ))
 ```
 
+After guard execution, read the verification result from the same context:
+
+```python
+from langchain_capiscio import get_capiscio_context
+
+ctx = get_capiscio_context()
+ctx.verified   # True if badge was valid
+ctx.claims     # Decoded badge claims dict
+ctx.warnings   # List of warning strings (monitor/log modes), or None
+```
+
 ---
 
 ## Full Example: A2A Agent with Trust + Events