chore: 清理 src 下 113 项未使用导入和死代码

删除未使用的文件（BuiltinStatusLine.tsx、4 个重复的 .ts stub）、
移除约 55 个文件中未使用的 React 导入、
清理约 50 处未使用的导入/变量/参数。
净减少 ~296 行代码，precheck 4077 测试全部通过。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/services/skillLearning/observationStore.ts

@@ -3,7 +3,6 @@ import { dirname, join } from 'node:path'
 import { createHash, randomUUID } from 'node:crypto'
 import type {
   SkillLearningProjectContext as BaseSkillLearningProjectContext,
-  SkillLearningScope,
   SkillObservation as BaseSkillObservation,
   SkillObservationEvent,
   SkillObservationOutcome,

src/services/skillLearning/toolEventObserver.ts

@@ -65,7 +65,7 @@ export function pruneEmittedTurns(): void {
   // Prune over-sized Sets first. FIFO by insertion order — NOT by turn
   // number magnitude. Non-monotonic turn ordering (e.g. replayed transcripts
   // or nested tool chains) should not cause us to evict the wrong entries.
-  for (const [sessionId, turns] of emittedTurns) {
+  for (const [_sessionId, turns] of emittedTurns) {
     if (turns.size > EMITTED_TURNS_SET_MAX) {
       const iter = turns.values()
       const toDrop = turns.size - EMITTED_TURNS_SET_KEEP