feat(a2a): SLIMRPC transport binding for A2A CLI

[Tehsmash]

Context

Closes part of #1786. Blocked by #1804.

Coding agents need to delegate tasks to sub-agents over SLIM. The A2A CLI is the natural client for this but currently has no SLIMRPC transport. This issue adds a SLIMRPC transport plugin so that when an Agent Card advertises SLIMRPC as a supported protocol binding, the A2A CLI can use SLIM as its underlying delivery mechanism, consuming the hierarchical config introduced in #1802.

Proposal

Add a SLIMRPC transport binding to the A2A CLI:

• The CLI selects the transport from the Agent Card's supported protocol bindings in priority order.
• A --prefer-transport slimrpc flag hints that SLIMRPC should be preferred if available in the Agent Card, without overriding cases where SLIMRPC is not advertised.
• The transport reads SLIM node address and identity from the hierarchical config (feat(slim-bindings): hierarchical config loading for SLIM node address and identity #1802).
• Single Agent Card → point-to-point SLIM session.
• Multiple Agent Cards → SLIMRPC multicast over a group session.
• Supports both streaming responses and task-ID polling.

Example usage

# Transport selected automatically from Agent Card priority order
a2a send agent-b.json '{"task": "..."}'

# Prefer SLIMRPC if the Agent Card supports it
a2a send --prefer-transport slimrpc agent-b.json '{"task": "..."}'

# Multicast — multiple agent cards
a2a send agent-b.json agent-c.json '{"task": "..."}'

# Stream responses
a2a send --stream agent-b.json '{"task": "..."}'

Acceptance Criteria

• SLIMRPC transport plugin implemented and wired into A2A CLI
• Transport selected from Agent Card supported bindings in priority order
• --prefer-transport flag implemented to influence selection without overriding card availability
• Config consumed from hierarchical config file / env vars (feat(slim-bindings): hierarchical config loading for SLIM node address and identity #1802)
• P2P session used for single Agent Card input
• Group session / multicast used for multiple Agent Cards
• Streaming and polling modes both supported
• Integration test demonstrating end-to-end A2A task over SLIM