Consolidate duplicate kill ring and timeout-race modules (CL-6790)

- Delete the dead Ink-era kill ring (src/tui/kill-ring.ts + test); the
  OpenTUI prompt-kill-ring.ts is the sole implementation.
- Extract shared timeout-race primitives (src/util/budget-race.ts) used
  by shell-guard's search budget and the tool-execution watchdog,
  replacing two copies of the same AbortController+setTimeout race and
  BUDGET_EXPIRED sentinel; both now reference the shared timeout-prefix
  string constant instead of a hardcoded literal.
- runtime-bridge.ts re-exports mapReactorLike from stream-event-map.ts
  instead of wrapping it in an identical local function.

Author: TheGreatAxios

CHANGELOG.md

@@ -11,6 +11,18 @@ matching `## [X.Y.Z]` section (plus install instructions). Do not maintain
 parallel copies under `docs/` or `scripts/notes/`. At cut time: rename
 `## [Unreleased]` to `## [X.Y.Z] - YYYY-MM-DD`, then run the release script.
 
+## [Unreleased]
+
+### Internal
+
+- Removed the dead Ink-era kill ring copy (`src/tui/kill-ring.ts`); the OpenTUI
+  prompt kill ring (`src/tui/prompt-kill-ring.ts`) is the sole implementation.
+- Extracted the shared timeout-race helper (`src/util/budget-race.ts`) used by
+  the shell-guard search budget and the tool-execution watchdog, replacing two
+  independent copies of the same `AbortController` + `setTimeout` race.
+- `runtime-bridge.ts` now re-exports `mapReactorLike` from `stream-event-map.ts`
+  instead of wrapping it in an identical local function.
+
 ## [0.2.108] - 2026-08-24
 
 ### Agent

src/plugins/shell-guard-plugin.ts

@@ -1,7 +1,8 @@
 import { spawn, type ChildProcess } from "node:child_process";
 import { realpathSync } from "node:fs";
 import type { ToolPlugin } from "@intx/tools-posix";
-import { formatSearchTimeoutMessage } from "./tool-time-budget.js";
+import { formatSearchTimeoutMessage, TIMEOUT_PREFIX } from "./tool-time-budget.js";
+import { BUDGET_EXPIRED, budgetExpiry, withTimeout } from "../util/budget-race.js";
 import type { ToolDefinition } from "@intx/types/runtime";
 import {
   assertShellCwdUsable,
@@ -325,38 +326,6 @@ function optionalNumber(value: unknown): number | undefined {
   return typeof value === "number" && Number.isFinite(value) ? value : undefined;
 }
 
-function withTimeout(
-  signal: AbortSignal,
-  timeoutMs: number,
-): { signal: AbortSignal; dispose: () => void } {
-  const controller = new AbortController();
-  const onParentAbort = () => controller.abort();
-  signal.addEventListener("abort", onParentAbort, { once: true });
-  const timer = setTimeout(() => controller.abort(), timeoutMs);
-  if (signal.aborted) controller.abort();
-  return {
-    signal: controller.signal,
-    dispose: () => {
-      clearTimeout(timer);
-      signal.removeEventListener("abort", onParentAbort);
-    },
-  };
-}
-
-const BUDGET_EXPIRED = Symbol("search-budget-expired");
-
-function budgetExpiry(signal: AbortSignal): Promise<typeof BUDGET_EXPIRED> {
-  return new Promise((resolve) => {
-    if (signal.aborted) {
-      resolve(BUDGET_EXPIRED);
-      return;
-    }
-    signal.addEventListener("abort", () => resolve(BUDGET_EXPIRED), {
-      once: true,
-    });
-  });
-}
-
 /**
  * Replaces stock run_shell with a hard-capped implementation, and applies a
  * 10s wall-clock budget to grep/search_files when the agent does not abort
@@ -510,7 +479,7 @@ export function shellGuardPlugin(
           if (
             outcome.isError === true &&
             typeof outcome.content === "string" &&
-            outcome.content.includes("[timed out before completing]")
+            outcome.content.includes(TIMEOUT_PREFIX)
           ) {
             return outcome;
           }

src/plugins/tool-time-budget.ts

@@ -2,7 +2,7 @@
 
 export type ScopedSearchTool = "grep" | "search_files";
 
-const TIMEOUT_PREFIX = "[timed out before completing]";
+export const TIMEOUT_PREFIX = "[timed out before completing]";
 
 export function scopedSearchRetryHints(tool: ScopedSearchTool): string {
   const base =

src/tui/kill-ring.test.ts

@@ -15,9 +15,8 @@
  * `rotateYank` hand back the text to splice in; shell.ts performs the splice
  * against the InputRenderable directly.
  *
- * Mirrors the semantics of src/tui/kill-ring.ts (the Ink reference) without
- * importing from it — the two prompt implementations are independent trees
- * during the OpenTUI cutover.
+ * Sole kill ring implementation (the former Ink-era src/tui/kill-ring.ts
+ * copy was retired once the OpenTUI cutover made it dead code).
  */
 
 export const KILL_RING_MAX = 10;

src/tui/kill-ring.ts

@@ -86,14 +86,15 @@ import {
   PRODUCTION_REACTOR_TYPES,
   createStreamMapContext,
   mapProductionEvent,
-  mapReactorLike as mapReactorLikeImpl,
+  mapReactorLike,
   type BridgeInboundEvent,
   type ReactorLikeEvent,
   type StreamMapContext,
 } from "./stream-event-map.js";
 
-/** Re-export map types so existing `from "./runtime-bridge"` imports keep working. */
+/** Re-export map types/fn so existing `from "./runtime-bridge"` imports keep working. */
 export type { BridgeInboundEvent, ReactorLikeEvent, StreamMapContext };
+export { mapReactorLike };
 
 /** Outbound actions the UI asks the session runtime to perform. */
 export interface SessionPort {
@@ -260,14 +261,6 @@ function isBridgeInbound(event: { type: string }): event is BridgeInboundEvent {
   }
 }
 
-/**
- * Map a reactor-like event into zero or more canonical bridge events.
- * Stateless (fixture-friendly). Live sessions use a StreamMapContext via handle.
- */
-export function mapReactorLike(event: ReactorLikeEvent): readonly BridgeInboundEvent[] {
-  return mapReactorLikeImpl(event);
-}
-
 function rowFromInbound(event: BridgeInboundEvent): StreamRow | null {
   switch (event.type) {
     case "user":