feat(a2a): investigate and implement A2A server with SLIMRPC inbound support for coding agents

[Tehsmash]

Context

Closes part of #1786.

For a coding agent to be reachable over SLIM — not just able to send — it needs a server-side component that accepts inbound A2A requests delivered via SLIMRPC. This issue covers the investigation and implementation of that component.

Investigation

Before building anything new, survey existing A2A server implementations (across coding agent harnesses and frameworks) to determine:

• Does a suitable A2A server already exist that could have SLIMRPC support added to it?
• What protocol does it use for local agent communication (ACP, stdio, HTTP)?
• What changes would be needed to add SLIMRPC as an additional inbound transport?

If no suitable existing implementation is found, define and build a minimal new one.

Requirements (regardless of implementation path)

• Can be started by or alongside a coding agent process
• Accepts inbound A2A tasks delivered over SLIMRPC
• Dispatches tasks to the local agent and returns results / streams responses
• Uses the hierarchical config (feat(slim-bindings): hierarchical config loading for SLIM node address and identity #1802) for SLIM node address and identity
• Exposes an Agent Card advertising SLIMRPC as a supported protocol binding

Acceptance Criteria

• Investigation completed and findings documented (existing project vs. new build decision)
• A2A server with SLIMRPC inbound transport implemented or extended
• Agent Card generated/served with SLIMRPC protocol binding populated
• Hierarchical config consumed for node address and identity
• Integration test: coding agent receives and responds to an inbound A2A task over SLIM