[FEATURE] Enable writable agent state and properties within hooks

[nagabharann]

Problem Statement

Agent state can only be modified outside of hooks (before agent.prompt()). Properties are also read-only within hooks. This prevents us from implementing pause/resume functionality and dynamic agent behavior modification from within hook handlers.

Proposed Solution

Provide a mechanism to modify agent state and properties from within hook handlers. This could be through:

1. Direct write access to agent state/properties via the event context
2. A method on the event object to update state (e.g., event.update_state(...))
3. A callback mechanism to safely modify agent internals

Use Case

We are building pause/resume functionality for long-running agents. When resuming a paused agent, we need to:

1. Rehydrate agent state from a persisted snapshot
2. Modify agent properties to reflect the resumed context
3. Update conversation history before continuing execution

Without writable state in hooks, we cannot implement this pattern cleanly.

Alternatives Solutions

We currently recreate agents from scratch when resuming, which is inefficient and loses runtime context that isn't serialized.

Additional Context

No response

[zastrowm]

@nagabharann what properties are you looking to update? All hooks get access to the Agent and thus any writable properties on the agent are writable from hooks. What properties are missing that you'd like to write.

Any properties encompassed in a snapshot I think would be achievable via #1138 but I want to ensure that there's not something that's missing