feat(flow): support custom persistence key in @persist (#5649)

* feat(flow): add optional key param to @persist decorator

Allows users to specify which state attribute to use as the
persistence key instead of always defaulting to state.id.

Usage: @persist(key='conversation_id')

Falls back to state.id when key is not provided (no breaking change).
Raises ValueError if the specified key is missing or falsy on state.

* docs(flow): document @persist key parameter for custom persistence keys

* fix(flow): use explicit None check for persist key to avoid empty-string fallback

---------

Co-authored-by: iris-clawd <iris-clawd@anthropic.com>
Co-authored-by: iris-clawd <iris@crewai.com>
Co-authored-by: Lorenze Jay <63378463+lorenzejay@users.noreply.github.com>

docs/en/concepts/flows.mdx

@@ -380,6 +380,33 @@ class AnotherFlow(Flow[dict]):
         print("Method-level persisted runs:", self.state["runs"])
 ```
 
+### Custom Persistence Key
+
+By default, `@persist` uses the auto-generated `state.id` field as the persistence key. If your flow models its own identifier — for example a `conversation_id` shared across sessions — you can pass a `key` argument and `@persist` will use that attribute as the flow UUID instead:
+
+```python
+from crewai.flow.flow import Flow, listen, start
+from crewai.flow.persistence import persist
+from pydantic import BaseModel
+
+class ConversationState(BaseModel):
+    conversation_id: str
+    turn: int = 0
+
+@persist(key="conversation_id")  # Use a custom field as the persistence key
+class ConversationFlow(Flow[ConversationState]):
+    @start()
+    def begin(self):
+        self.state.turn += 1
+        print(f"Conversation {self.state.conversation_id} turn {self.state.turn}")
+
+# Resuming the same conversation reloads its prior state by conversation_id
+flow = ConversationFlow(conversation_id="user-42")
+flow.kickoff()
+```
+
+The decorator reads the value at `state[key]` for dict states, or `getattr(state, key)` for Pydantic / object states. If the named attribute is missing or falsy at save time, `@persist` raises a `ValueError` such as `Flow state is missing required persistence key 'conversation_id'`. When `key` is omitted, the existing behavior is preserved and `state.id` is used.
+
 ### How It Works
 
 1. **Unique State Identification**

docs/en/concepts/production-architecture.mdx

@@ -146,6 +146,15 @@ class ProductionFlow(Flow[AppState]):
     # ...
 ```
 
+By default `@persist` keys saved state by the auto-generated `state.id`. If your application already has a natural identifier — for example a `conversation_id` that ties multiple runs to the same user session — pass it as `key` and the decorator will use that attribute as the flow UUID. A `ValueError` is raised if the named attribute is missing or falsy at save time.
+
+```python
+@persist(key="conversation_id")
+class ProductionFlow(Flow[AppState]):
+    # AppState must expose conversation_id; resuming a session reloads its prior state
+    ...
+```
+
 ## Summary
 
 - **Start with a Flow.**