feat(tui): add subagent inbox foundation

Preserve the subagent inbox foundation behavior on the current origin/main base and collapse the branch back to a single commit for easier future restacks.

codex-rs/protocol/src/protocol.rs

@@ -98,6 +98,27 @@ pub const COLLABORATION_MODE_CLOSE_TAG: &str = "</collaboration_mode>";
 pub const REALTIME_CONVERSATION_OPEN_TAG: &str = "<realtime_conversation>";
 pub const REALTIME_CONVERSATION_CLOSE_TAG: &str = "</realtime_conversation>";
 pub const USER_MESSAGE_BEGIN: &str = "## My request for Codex:";
+pub const AGENT_INBOX_KIND: &str = "agent_inbox";
+pub const AGENT_INBOX_MESSAGE_PREFIX: &str = "[agent_inbox:";
+
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema)]
+pub struct AgentInboxPayload {
+    pub injected: bool,
+    pub kind: String,
+    pub sender_thread_id: ThreadId,
+    pub message: String,
+}
+
+impl AgentInboxPayload {
+    pub fn new(sender_thread_id: ThreadId, message: String) -> Self {
+        Self {
+            injected: true,
+            kind: AGENT_INBOX_KIND.to_string(),
+            sender_thread_id,
+            message,
+        }
+    }
+}
 
 /// Submission Queue Entry - requests from user
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema)]
@@ -3396,6 +3417,16 @@ pub struct CollabAgentSpawnBeginEvent {
     pub reasoning_effort: ReasoningEffortConfig,
 }
 
+#[derive(Debug, Clone, Copy, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS, Default)]
+#[serde(rename_all = "snake_case")]
+#[ts(rename_all = "snake_case")]
+pub enum AgentSpawnMode {
+    #[default]
+    Spawn,
+    Fork,
+    Watchdog,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
 pub struct CollabAgentRef {
     /// Thread ID of the receiver/new agent.
@@ -3443,6 +3474,9 @@ pub struct CollabAgentSpawnEndEvent {
     pub model: String,
     /// Effective reasoning effort used by the spawned agent after inheritance and role overrides.
     pub reasoning_effort: ReasoningEffortConfig,
+    /// Spawn mode used for this agent.
+    #[serde(default)]
+    pub spawn_mode: AgentSpawnMode,
     /// Last known status of the new agent reported to the sender agent.
     pub status: AgentStatus,
 }