fix(shared): GatewayEventMap drops phantom keys and types child_session_id

Re-verified against the tui_gateway emitters:
- SubagentEventPayload.cost_usd / .iteration: not in
  tool_progress.py::_SUBAGENT_FIELDS, never emitted → removed; the TUI's
  turnController no longer copies them (its SubagentProgress keeps the
  fields for spawn-history persistence).
- SubagentEventPayload.child_session_id: emitted (in _SUBAGENT_FIELDS, read
  by agent_callbacks.py::_mirror_subagent_to_child) but untyped → added.
- ToolCompletePayload.error: _on_tool_complete never sets it → removed;
  the TUI's completeTool drops its dead `error` parameter and renders the
  trail line as non-error (which is what it always did on the wire).
- ToolStartPayload.todos: not on the wire either, but the TUI handler and
  its fixtures exercise recordTodos from tool.start; kept with a comment
  saying so rather than churning the handler.
- MessageCompletePayload.failure_reason: prompt_turn.py passes
  result.get("failure_reason") through → `string | null`.

apps/shared/src/gateway-events.ts

@@ -64,7 +64,8 @@ export type SubagentStatus = 'completed' | 'error' | 'failed' | 'interrupted' |
 /** `tui_gateway/tool_progress.py::_progress_subagent` — every `subagent.*` frame. */
 export interface SubagentEventPayload {
   api_calls?: number
-  cost_usd?: number
+  /** The child's own gateway session id — the key a watch window mirrors. */
+  child_session_id?: string
   /** Batch (delegation) id this subagent belongs to — distinguishes
    *  interleaved `[n/N]` progress from concurrent or nested fan-outs. */
   delegation_id?: string
@@ -74,7 +75,6 @@ export interface SubagentEventPayload {
   files_written?: string[]
   goal: string
   input_tokens?: number
-  iteration?: number
   model?: string
   output_tail?: { is_error?: boolean; preview?: string; tool?: string }[]
   output_tokens?: number
@@ -115,7 +115,7 @@ export interface MessageCompletePayload {
   /** `status: "error"` — the failure message (`text` may be streamed output). */
   error?: string
   error_surface?: ErrorSurface
-  failure_reason?: string
+  failure_reason?: string | null
   /** `status: "error"` — `text` is streamed partial output to keep, not the error string. */
   partial?: boolean
   reasoning?: string
@@ -142,6 +142,8 @@ export interface ToolStartPayload {
   /** Mirrored child tool rows carry a short preview instead of args. */
   preview?: string
   tool_id: string
+  /** Not on the wire (`_on_tool_start` never sets it): the todo snapshot rides `tool.complete` /
+   *  `todo.updated`. Kept because the TUI handler reads it and its fixtures exercise that path. */
   todos?: unknown[]
 }
 
@@ -149,7 +151,6 @@ export interface ToolStartPayload {
 export interface ToolCompletePayload {
   args?: Record<string, unknown>
   duration_s?: number
-  error?: string
   inline_diff?: string
   name?: string
   /** Parsed JSON when the tool returned JSON, else the raw string. */

ui-tui/src/app/createGatewayEventHandler.ts

@@ -1239,7 +1239,6 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
             inlineDiffText,
             ev.payload.tool_id,
             ev.payload.name,
-            ev.payload.error,
             ev.payload.duration_s,
             resultText
           )
@@ -1247,7 +1246,6 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
           turnController.recordToolComplete(
             ev.payload.tool_id,
             ev.payload.name,
-            ev.payload.error,
             ev.payload.summary,
             ev.payload.duration_s,
             ev.payload.todos,

ui-tui/src/app/turnController.ts

@@ -796,7 +796,6 @@ class TurnController {
   recordToolComplete(
     toolId: string,
     fallbackName?: string,
-    error?: string,
     summary?: string,
     duration?: number,
     todos?: unknown,
@@ -807,38 +806,26 @@ class TurnController {
     }
 
     this.recordTodos(todos)
-    const line = this.completeTool(toolId, fallbackName, error, summary, duration, resultText)
+    const line = this.completeTool(toolId, fallbackName, summary, duration, resultText)
 
     this.pendingSegmentTools = [...this.pendingSegmentTools, line]
     this.flushPendingToolsIntoLastSegment()
     this.publishToolState()
   }
 
-  recordInlineDiffToolComplete(
-    diffText: string,
-    toolId: string,
-    fallbackName?: string,
-    error?: string,
-    duration?: number,
-    resultText?: string
-  ) {
+  recordInlineDiffToolComplete(diffText: string, toolId: string, fallbackName?: string, duration?: number, resultText?: string) {
     if (this.interrupted) {
       return
     }
 
     this.flushStreamingSegment()
-    this.pushInlineDiffSegment(diffText, [this.completeTool(toolId, fallbackName, error, '', duration, resultText)])
+    this.pushInlineDiffSegment(diffText, [this.completeTool(toolId, fallbackName, '', duration, resultText)])
     this.publishToolState()
   }
 
-  private completeTool(
-    toolId: string,
-    fallbackName?: string,
-    error?: string,
-    summary?: string,
-    duration?: number,
-    resultText?: string
-  ) {
+  // `tool.complete` carries no error flag on the wire (tui_gateway/tool_progress.py::_on_tool_complete);
+  // a failed tool surfaces through its result text, so every trail line renders as non-error.
+  private completeTool(toolId: string, fallbackName?: string, summary?: string, duration?: number, resultText?: string) {
     const done = this.activeTools.find(tool => tool.id === toolId)
     const name = done?.name ?? fallbackName ?? 'tool'
     const label = toolTrailLabel(name)
@@ -849,18 +836,12 @@ class TurnController {
         ? buildVerboseToolTrailLine(
             name,
             done?.context || '',
-            Boolean(error),
+            false,
             duration ?? fallbackDuration,
             done?.verboseArgs,
-            error || resultText || summary || ''
-          )
-        : buildToolTrailLine(
-            name,
-            done?.context || '',
-            Boolean(error),
-            error || summary || '',
-            duration ?? fallbackDuration
+            resultText || summary || ''
           )
+        : buildToolTrailLine(name, done?.context || '', false, summary || '', duration ?? fallbackDuration)
 
     this.activeTools = this.activeTools.filter(tool => tool.id !== toolId)
 
@@ -1044,14 +1025,12 @@ class TurnController {
       const next: SubagentProgress = {
         ...base,
         apiCalls: p.api_calls ?? base.apiCalls,
-        costUsd: p.cost_usd ?? base.costUsd,
         delegationId: p.delegation_id ?? base.delegationId,
         depth: p.depth ?? base.depth,
         filesRead: p.files_read ?? base.filesRead,
         filesWritten: p.files_written ?? base.filesWritten,
         goal: p.goal || base.goal,
         inputTokens: p.input_tokens ?? base.inputTokens,
-        iteration: p.iteration ?? base.iteration,
         model: p.model ?? base.model,
         outputTail,
         outputTokens: p.output_tokens ?? base.outputTokens,