feat(agent): add remaining client→server messages and ClientMessage enum

  Adds the seven non-Settings client-to-server messages
  (UpdateSpeak, UpdateThink, UpdatePrompt, InjectUserMessage,
  InjectAgentMessage, FunctionCallResponse, KeepAlive) plus a
  unified ClientMessage envelope over all eight.

  UpdateSpeak / UpdateThink reuse the one-or-many serde helpers from
  InlineAgentConfig: a single provider serializes as a scalar object,
  multiple as an array.

  InjectAgentMessage carries an optional behavior field
  (InjectAgentBehavior::{Default, Queue}) controlling how injection
  interacts with an in-progress turn.

  FunctionCallResponseMessage is bidirectional in the spec — `id` is
  optional so the same struct can serve both client→server replies
  (id required) and server→client reports (id may be omitted).

  ClientMessage uses #[serde(untagged)] so each inner struct keeps
  its own `type` field via a single-variant marker enum, allowing
  standalone (de)serialization. The `large_enum_variant` lint is
  allowed at the enum level: messages are constructed and immediately
  sent (or deserialized and immediately matched), never stored, so
  boxing would add a heap allocation per WebSocket send.

  16 new round-trip serde tests, 104 passing in the agent module.
  The server-to-client AgentResponse enum and the WebSocket
  connection helpers follow in subsequent commits.

Author: GregHolmes