refactor(hooks): remove beast-mode system integration

Remove the beast-mode-system hook and all transform wiring so Copilot-specific prompt injection is fully eliminated from the runtime pipeline.

src/hooks/beast-mode-system/hook.test.ts

@@ -1,54 +0,0 @@
-import { describe, expect, test } from "bun:test"
-import { clearSessionModel, setSessionModel } from "../../shared/session-model-state"
-import { createBeastModeSystemHook, BEAST_MODE_SYSTEM_PROMPT } from "./hook"
-
-describe("beast-mode-system hook", () => {
-  test("injects beast mode prompt for copilot gpt-4.1", async () => {
-    //#given
-    const sessionID = "ses_beast"
-    setSessionModel(sessionID, { providerID: "github-copilot", modelID: "gpt-4.1" })
-    const hook = createBeastModeSystemHook()
-    const output = { system: [] as string[] }
-
-    //#when
-    await hook["experimental.chat.system.transform"]?.({ sessionID }, output)
-
-    //#then
-    expect(output.system[0]).toContain("Beast Mode")
-    expect(output.system[0]).toContain(BEAST_MODE_SYSTEM_PROMPT.trim().slice(0, 20))
-
-    clearSessionModel(sessionID)
-  })
-
-  test("does not inject for other models", async () => {
-    //#given
-    const sessionID = "ses_no_beast"
-    setSessionModel(sessionID, { providerID: "anthropic", modelID: "gpt-5.3-codex" })
-    const hook = createBeastModeSystemHook()
-    const output = { system: [] as string[] }
-
-    //#when
-    await hook["experimental.chat.system.transform"]?.({ sessionID }, output)
-
-    //#then
-    expect(output.system.length).toBe(0)
-
-    clearSessionModel(sessionID)
-  })
-
-  test("avoids duplicate insertion", async () => {
-    //#given
-    const sessionID = "ses_dupe"
-    setSessionModel(sessionID, { providerID: "github-copilot", modelID: "gpt-4.1" })
-    const hook = createBeastModeSystemHook()
-    const output = { system: [BEAST_MODE_SYSTEM_PROMPT] }
-
-    //#when
-    await hook["experimental.chat.system.transform"]?.({ sessionID }, output)
-
-    //#then
-    expect(output.system.length).toBe(1)
-
-    clearSessionModel(sessionID)
-  })
-})

src/hooks/beast-mode-system/hook.ts

@@ -1,31 +0,0 @@
-import { getSessionModel } from "../../shared/session-model-state"
-
-export const BEAST_MODE_SYSTEM_PROMPT = `Beast Mode (Copilot GPT-4.1)
-
-You are an autonomous coding agent. Execute the task end-to-end.
-- Make a brief plan, then act.
-- Prefer concrete edits and verification over speculation.
-- Run relevant tests when feasible.
-- Do not ask the user to perform actions you can do yourself.
-- If blocked, state exactly what is needed to proceed.
-- Keep responses concise and actionable.`
-
-function isBeastModeModel(model: { providerID: string; modelID: string } | undefined): boolean {
-  return model?.providerID === "github-copilot" && model.modelID === "gpt-4.1"
-}
-
-export function createBeastModeSystemHook() {
-  return {
-    "experimental.chat.system.transform": async (
-      input: { sessionID: string },
-      output: { system: string[] },
-    ): Promise<void> => {
-      const model = getSessionModel(input.sessionID)
-      if (!isBeastModeModel(model)) return
-
-      if (output.system.some((entry) => entry.includes("Beast Mode"))) return
-
-      output.system.unshift(BEAST_MODE_SYSTEM_PROMPT)
-    },
-  }
-}

src/hooks/beast-mode-system/index.ts

@@ -1 +0,0 @@
-export { createBeastModeSystemHook, BEAST_MODE_SYSTEM_PROMPT } from "./hook"

src/hooks/index.ts

@@ -49,5 +49,4 @@ export { createTasksTodowriteDisablerHook } from "./tasks-todowrite-disabler";
 export { createRuntimeFallbackHook, type RuntimeFallbackHook, type RuntimeFallbackOptions } from "./runtime-fallback";
 export { createWriteExistingFileGuardHook } from "./write-existing-file-guard";
 export { createHashlineReadEnhancerHook } from "./hashline-read-enhancer";
-export { createBeastModeSystemHook, BEAST_MODE_SYSTEM_PROMPT } from "./beast-mode-system";
 export { createJsonErrorRecoveryHook, JSON_ERROR_TOOL_EXCLUDE_LIST, JSON_ERROR_PATTERNS, JSON_ERROR_REMINDER } from "./json-error-recovery";