feat: feat(cluster-protocol): add capability-gated read-only agent attach

[tomdps]

Closes #749

[greptile-apps]

Greptile Summary

Adds capability-advertised, read-only agent/attach subscriptions across the protocol, server, client, testkit, and generated protocol artifacts.

• Defines bounded attach parameters, results, events, and close notifications.
• Adds in-process and NDJSON clients with shared cursorless subscription handling.
• Adds server-side admission, streaming, cancellation, overflow, and error mapping.
• Extends capability vectors, schemas, OpenRPC output, fixtures, and integration tests.

Confidence Score: 3/5

The encoding-failure subscription path needs to notify or disconnect the client before this PR is safe to merge.

An unencodable attachment event stops the server-side forwarding loop without a terminal notification, leaving the client-side subscription registered and waiting indefinitely.

crates/openengine-cluster-server/src/stdio/subscription.rs

Important Files Changed

Filename	Overview
crates/openengine-cluster-protocol/src/agent_attach.rs	Defines bounded attach wire types and validation consistently with the generated schemas and fixtures.
crates/openengine-cluster-server/src/agent_attach.rs	Adds transport-neutral attach establishment and backend delegation with bounded subscription queues.
crates/openengine-cluster-server/src/stdio/agent_attach.rs	Adds NDJSON attach dispatch using shared subscription machinery, including an encoding-failure path that terminates without notifying the client.
crates/openengine-cluster-server/src/stdio/subscription.rs	Extracts shared cursorless subscription handling but silently exits on event encoding failure without emitting a close notification.
crates/openengine-cluster-client/src/ndjson_subscription.rs	Generates common typed NDJSON subscription clients whose streams remain waiting when the server exits without a close notification.
crates/openengine-cluster-client/src/ndjson_agent_attach.rs	Instantiates the shared NDJSON subscription client for the new agent/attach method.
crates/openengine-cluster-client/src/lib.rs	Generalizes subscription opening so JSON-RPC establishment errors can propagate as typed client errors.
crates/openengine-cluster-server/tests/agent_attach.rs	Covers unsupported, unknown, inactive, oversized, cancellation, and capability-related attach behavior, including server survival after an oversized event.

Sequence Diagram

sequenceDiagram
    participant C as Client
    participant T as NDJSON Transport
    participant S as Cluster Server
    participant B as Backend
    C->>T: agent/attach(execution)
    T->>S: JSON-RPC request
    S->>B: agent_attach(context, params, capacity)
    alt unknown or inactive execution
        B-->>S: BackendError
        S-->>T: JSON-RPC error
        T-->>C: Typed RPC error
    else attachment established
        B-->>S: result, event stream, handle
        S-->>T: AgentAttachResult(subscriptionId)
        T-->>C: result and event stream
        loop live attachment events
            B-->>S: Working / Output / Settled
            S-->>T: event notification
            T-->>C: typed event
        end
        C->>T: subscription/cancel
        T->>S: cancel notification
        S->>B: release attachment handle
    end

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_{Reviews (1): Last reviewed commit: "feat: implement #749 - feat(cluster-prot..." | Re-trigger Greptile}

[greptile-apps]

Encoding failure leaves stream open

When an attachment event cannot be encoded, converting the error to None ends the server subscription without sending subscription/closed; the client transport retains its registered sender and next() waits indefinitely instead of observing termination.