Add peer-agent context to agents/chat client_context

[chubes4]

Problem

The canonical agents/chat ability is the right primitive for agent-to-agent chat, but its client_context.source enum currently covers channel/bridge/rest/block and does not have a first-class peer-agent source.

Data Machine can route peer turns through agents/chat today, but it has to describe the request as source=bridge to stay compatible with the current schema. That works, but it blurs the distinction between a user-facing bridge and an agent-to-agent turn.

Desired behavior

• agents/chat should support a first-class peer-agent context, e.g. client_context.source = agent or peer_agent.
• The schema should document peer-agent metadata such as caller agent, call depth, root request/correlation IDs, and whether the turn is a delegated sub-question.
• Consumers can use that context for observability, loop prevention, transcript labeling, and policy.

Suggested implementation shape

• Extend the client_context.source enum with an agent/peer value.
• Add optional schema fields for peer calls, such as caller_agent, peer_agent_call, agent_chat_depth, and caller_session_id.
• Keep the canonical agents/chat contract as the only chat primitive; do not add a parallel ask-agent ability.

Related

• Data Machine PR using canonical agents/chat as an opt-in peer tool: Add consult agent tool Extra-Chill/data-machine#2062