Typed host-event injection

Status: Harn extension

Harn advertises session/inject_host_event under agentCapabilities.session.injectHostEvent. The request carries a session ID and the same typed event contract used by the in-process runtime:

{
  "sessionId": "session-1",
  "event": {
    "kind": "host_tool_result",
    "delivery": "turn_boundary",
    "payload": {
      "tool_name": "host.search",
      "raw_input": { "query": "typed injection" },
      "raw_output": { "matches": 3 }
    },
    "provenance": {
      "initiator": "user",
      "source": "ide",
      "ts_ms": 1773298800000
    }
  }
}

Supported kinds are host_tool_result and host_attachment. Supported delivery seams are immediate, turn_boundary, and after_next_tool_call. Harn validates, sanitizes, records, queues, and renders the event; hosts cannot supply rendered prompt material. Unknown fields are rejected so misspelled provenance or delivery policy cannot silently degrade.

Successful responses contain the durable injection_id, monotonic sequence, resolved delivery, and status. Unknown sessions use JSON-RPC error -32004; invalid request or payload contracts use -32602.

The generated TypeScript, Swift, Python, Go, and Rust artifacts publish this method vocabulary. Typed request structures are included in every generated language except the dependency-free Rust vocabulary module, where embedders use harn_serve::adapters::acp::AcpSessionInjectHostEventParams directly.