fix: 修复 BriefTool 循环依赖导致 isBriefEnabled 未定义

将模块顶层 require() 改为懒加载函数 getBriefToolModule()，
延迟到实际调用时才加载模块，避免循环依赖时模块尚未完成初始化。

(cherry picked from commit d4a601475f)

src/constants/prompts.ts

@@ -63,7 +63,6 @@ import { getAntModelOverrideConfig } from '../utils/model/antModels.js'
 import { isMcpInstructionsDeltaEnabled } from '../utils/mcpInstructionsDelta.js'
 import { BRIEF_PROACTIVE_SECTION as BRIEF_PROACTIVE_SECTION_VALUE } from '../tools/BriefTool/prompt.js'
 import * as proactiveModuleValue from '../proactive/index.js'
-import * as briefToolModuleValue from '../tools/BriefTool/BriefTool.js'
 import { DISCOVER_SKILLS_TOOL_NAME as DISCOVER_SKILLS_TOOL_NAME_VALUE } from '../tools/DiscoverSkillsTool/prompt.js'
 import * as skillSearchFeatureCheckValue from '../services/skillSearch/featureCheck.js'
 
@@ -81,10 +80,11 @@ const BRIEF_PROACTIVE_SECTION: string | null =
   feature('KAIROS') || feature('KAIROS_BRIEF')
     ? BRIEF_PROACTIVE_SECTION_VALUE
     : null
-const briefToolModule =
-  feature('KAIROS') || feature('KAIROS_BRIEF')
-    ? briefToolModuleValue
+function getBriefToolModule() {
+  return feature('KAIROS') || feature('KAIROS_BRIEF')
+    ? (require('../tools/BriefTool/BriefTool.js') as typeof import('../tools/BriefTool/BriefTool.js'))
     : null
+}
 const DISCOVER_SKILLS_TOOL_NAME: string | null = feature(
   'EXPERIMENTAL_SKILL_SEARCH',
 )
@@ -846,7 +846,7 @@ function getBriefSection(): string | null {
   // Whenever the tool is available, the model is told to use it. The
   // /brief toggle and --brief flag now only control the isBriefOnly
   // display filter — they no longer gate model-facing behavior.
-  if (!briefToolModule?.isBriefEnabled()) return null
+  if (!getBriefToolModule()?.isBriefEnabled()) return null
   // When proactive is active, getProactiveSection() already appends the
   // section inline. Skip here to avoid duplicating it in the system prompt.
   if (
@@ -910,5 +910,5 @@ Do not narrate each step, list every file you read, or explain routine actions.
 
 The user context may include a \`terminalFocus\` field indicating whether the user's terminal is focused or unfocused. Use this to calibrate how autonomous you are:
 - **Unfocused**: The user is away. Lean heavily into autonomous action — make decisions, explore, commit, push. Only pause for genuinely irreversible or high-risk actions.
-- **Focused**: The user is watching. Be more collaborative — surface choices, ask before committing to large changes, and keep your output concise so it's easy to follow in real time.${BRIEF_PROACTIVE_SECTION && briefToolModule?.isBriefEnabled() ? `\n\n${BRIEF_PROACTIVE_SECTION}` : ''}`
+- **Focused**: The user is watching. Be more collaborative — surface choices, ask before committing to large changes, and keep your output concise so it's easy to follow in real time.${BRIEF_PROACTIVE_SECTION && getBriefToolModule()?.isBriefEnabled() ? `\n\n${BRIEF_PROACTIVE_SECTION}` : ''}`
 }