feat(memory): add ConversationMemory trait and InMemoryConversation

[Mattbusel]

Closes #373

This PR implements the ConversationMemory abstraction proposed in issue #373.

Problem

Rig agents are currently stateless per call — chat() takes a Vec<Message> that the caller must manage manually. There is no standard way to plug in a memory backend. Issue #373 has been open since early 2025; the proposed interface was a MemoryStrategy trait with fetch_memory / insert_memory. This PR provides a concrete implementation.

What this adds

rig::memory::ConversationMemory trait

#[async_trait]
pub trait ConversationMemory: Send + Sync {
    async fn push(&self, message: Message);
    async fn history(&self) -> Vec<Message>;
    async fn clear(&self);
    async fn len(&self) -> usize;
    async fn is_empty(&self) -> bool { ... }  // default
}

All methods take &self (not &mut self) so implementations are trivially wrappable in Arc and shareable across agent handles and tokio tasks.

InMemoryConversation

Bounded VecDeque — keeps the last N messages, evicting oldest on overflow:

let memory = InMemoryConversation::new(20); // keep last 20 messages

memory.push(Message::user("What is backpressure?")).await;
memory.push(Message::assistant("Backpressure is …")).await;

let history = memory.history().await;
// pass history to agent.chat(prompt, history).await

• InMemoryConversation::unbounded() retains every message
• SlidingWindowMemory type alias for explicit window-size semantics

NoMemory

No-op default — identical to current Agent behaviour. Useful as a default type parameter so memory is opt-in with zero overhead:

pub struct MyAgent<M: ConversationMemory = NoMemory> { ... }

Custom backends

Any storage backend just implements the trait:

#[async_trait]
impl ConversationMemory for RedisMemory {
    async fn push(&self, msg: Message) { /* RPUSH */ }
    async fn history(&self) -> Vec<Message> { /* LRANGE */ vec![] }
    async fn clear(&self) { /* DEL */ }
    async fn len(&self) -> usize { /* LLEN */ 0 }
}

Tests (6)

• push_and_retrieve — basic store/fetch
• capacity_evicts_oldest — overflow eviction
• clear_empties_store
• unbounded_keeps_all — 200 messages, none dropped
• no_memory_always_empty — NoMemory is truly stateless
• shared_via_arc — two Arc handles, writes visible to both

What this does NOT do (intentional scope)

This PR adds the trait and the in-process implementation. Wiring ConversationMemory into AgentBuilder and Agent::chat() is a separate concern that touches more of the API surface — happy to follow up in a second PR once the trait shape is agreed on.

Designed and implemented by Matthew Busel.