✨ feat(vscode): 新增「了解一块代码」：AI 找功能代码占一栏，对话里接着改

主栏标题栏的放大镜问要了解哪一块，占一个栏位建一栏（和收藏共用 30 个栏位，视图改名 slot1..30，按 slotKindN 分收藏 / 功能挂标题栏按钮），同时开对话页让 Claude 只读地在全部工作区文件夹里找，按 --json-schema 交回环节 → 文件 + 符号 + 一句话。
对话复用预览顶部的聊天（CodeQa.askTop：会话续接、进行中的一轮、记录存 workspaceState）；结构不合格算这一轮没答成、留着能再画出来。
条目每次画、点时按眼下的代码找：有语言插件的解析成代码块按「类名.方法名」找，没有的问 VS Code 文档符号；找不到画成灰的。

vscode/src/codeReader/editor/previewEditor.ts

@@ -1,13 +1,13 @@
-import type { CustomTextEditorProvider, Disposable, DocumentSymbol, TextDocument, TextDocumentShowOptions, WebviewPanel } from 'vscode'
+import type { CustomTextEditorProvider, Disposable, TextDocument, TextDocumentShowOptions, WebviewPanel } from 'vscode'
 import type { ClaudeImage } from '../../cc/spawnClaude'
 import type { IndexFind } from '../callTree/endpoint'
 import type { CommentEdit, CommentJob, WriteResult } from '../commentRewrite'
-import type { BlockKind, LocalSymbol, RuleLocation, RuleReference, SourceRange } from '../contract'
+import type { BlockKind, LocalSymbol, RuleLocation, RuleReference } from '../contract'
 import type { DiffBaseSource } from '../diff/diffBase'
 import type { OldDocument, SentDocument } from '../diff/previewDiff'
 import type { WorkspaceEntries } from '../entries/workspaceEntries'
 import type { DefinitionStart, IndexedName, NameSources } from '../nameResolution'
-import type { CodeQa } from '../qa/codeQa'
+import type { CodeQa, TopRequest } from '../qa/codeQa'
 import type { CodeReaderRegistry } from '../registry'
 import type { LookupSources } from '../ruleLookups'
 import type { TreeSitterRuntime } from '../runtime'
@@ -16,6 +16,7 @@ import type { BlockFilter } from './blockFilter'
 import type { FoundLocation } from './definitions'
 import type { DefinitionPeek, ExtensionToWebview, Occurrence, Reveal, WebviewToExtension } from './messages'
 import type { TargetSpot } from './openTarget'
+import type { TopSubject } from './pathPreview'
 import { realpath } from 'node:fs/promises'
 import { commands, EndOfLine, Position, Range, Uri, window, workspace, WorkspaceEdit } from 'vscode'
 import { logger } from '../../logging/logger'
@@ -38,10 +39,11 @@ import { definitionSiteNames, namesOnCard, peekBlock } from './definitionPeek'
 import { definitionKind, findDefinitions, languageDefinitions, pickDefinition, ruleDefinitions } from './definitions'
 import { DefinitionSources } from './definitionSources'
 import { documentMessage } from './documentMessage'
+import { documentSymbolNodes } from './documentSymbols'
 import { NavigationContext, OCCURRENCES_CONTEXT } from './occurrenceNavigation'
 import { occurrencesOf } from './occurrences'
 import { languageOf, openingFor } from './openTarget'
-import { pathPreviewOf } from './pathPreview'
+import { PATH_PREVIEW_SCHEME, topSubjectOf } from './pathPreview'
 import { findRepoSymbol, isRepoImagePath, repoRootOf, resolveRepoEntry } from './repoPath'
 
 export const PREVIEW_VIEW_TYPE = 'superpowers.codeReader.preview'
@@ -81,6 +83,8 @@ export class PreviewEditorProvider implements CustomTextEditorProvider {
     readonly entries: Pick<WorkspaceEntries, 'namesOf' | 'onDidChange'>,
     /** 在代码阅读侧栏的树上定位一个目录（仓库链接指到目录时）；树上没有它时返回 false */
     readonly revealInTree: (uri: Uri) => Promise<boolean>,
+    /** 「了解一块代码」的对话页里接着说（subject 是那个功能，见 featureSubjectUri）；说不了时返回给人看的原因 */
+    readonly askFeature: (subject: Uri, request: TopRequest) => Promise<string | undefined>,
   ) {}
 
   /** 侧栏「问 AI」打开预览，并把光标放进顶部输入框。已经开着的就揭到前面再聚焦。 */
@@ -103,7 +107,7 @@ export class PreviewEditorProvider implements CustomTextEditorProvider {
 
   resolveCustomTextEditor(document: TextDocument, panel: WebviewPanel): void {
     // 问答带的图片（工作区文件夹的 .spx/code-qa/）也要能读，历史里才画得出缩略图
-    const imageRoot = this.qa.imageRoot(pathPreviewOf(document.uri)?.uri ?? document.uri)
+    const imageRoot = this.qa.imageRoot(topSubjectOf(document.uri).uri)
     panel.webview.options = { enableScripts: true, localResourceRoots: [webviewUiRoot(this.extensionUri), ...(imageRoot ? [imageRoot] : [])] }
     panel.webview.html = webviewHtml(panel.webview, this.extensionUri, 'code-reader.html')
     const key = document.uri.toString()
@@ -339,7 +343,8 @@ class Preview {
       return
     const request = ++this.latest
     this.callTrees.cancel()
-    if (pathPreviewOf(this.document.uri)) {
+    // 路径页、功能对话页：文档是空的，只在顶部聊天
+    if (this.document.uri.scheme === PATH_PREVIEW_SCHEME) {
       this.sent = undefined
       const reveal = this.pendingReveal
       this.pendingReveal = undefined
@@ -497,9 +502,9 @@ class Preview {
     this.post(this.owner.qa.view(subject.uri, file => this.panel.webview.asWebviewUri(file).toString(), subject.kind))
   }
 
-  /** 这一页问的是哪个文件或文件夹。路径页还原成真实路径，源码预览就是文档自己。 */
-  private subject(): { uri: Uri, kind: 'file' | 'folder' } {
-    return pathPreviewOf(this.document.uri) ?? { uri: this.document.uri, kind: 'file' }
+  /** 这一页顶部问的是什么（见 topSubjectOf）。 */
+  private subject(): { uri: Uri, kind: TopSubject } {
+    return topSubjectOf(this.document.uri)
   }
 
   /** 把光标放进顶部输入框；页面还没准备好就等 ready。 */
@@ -526,7 +531,10 @@ class Preview {
   private async ask(askId: string, anchor: string, question: string, images: ClaudeImage[]): Promise<void> {
     const subject = this.subject()
     if (anchor === PATH_ASK_ANCHOR) {
-      const refused = await this.owner.qa.askPath(subject.uri, subject.kind, { askId, question, images })
+      const request = { askId, question, images }
+      const refused = subject.kind === 'feature'
+        ? await this.owner.askFeature(subject.uri, request)
+        : await this.owner.qa.askPath(subject.uri, subject.kind, request)
       if (refused !== undefined)
         this.post({ type: 'answerEnd', askId, status: 'failed', error: refused })
       else
@@ -808,7 +816,7 @@ class Preview {
     const root = await repoRootOf(this.document.uri.fsPath)
     const asked = new Map<string, Promise<SymbolNode[]>>()
     const symbolsOf = (uri: string): Promise<SymbolNode[]> => {
-      const known = asked.get(uri) ?? this.symbolNodes(uri)
+      const known = asked.get(uri) ?? documentSymbolNodes(uri)
       asked.set(uri, known)
       return known
     }
@@ -919,29 +927,15 @@ class Preview {
 
   /** 用文档符号按名字找（`Agent`、`Agent.owner_id`）；没有这个名字就没有，不打开到文件顶上。 */
   private async locateRepoSymbol(uri: Uri, symbol: string): Promise<LinkTarget | undefined> {
-    const found = findRepoSymbol(await this.symbolNodes(uri.toString()), symbol)
+    const found = findRepoSymbol(await documentSymbolNodes(uri.toString()), symbol)
     return found && { uri: uri.toString(), selection: found.selection }
   }
 
-  /** 一个文件的文档符号（vscode.executeDocumentSymbolProvider）：先把文件读进来（不显示），范围换成那个文件里的下标。 */
-  private async symbolNodes(uri: string): Promise<SymbolNode[]> {
-    const doc = await workspace.openTextDocument(Uri.parse(uri))
-    const symbols = await commands.executeCommand<DocumentSymbol[] | undefined>('vscode.executeDocumentSymbolProvider', doc.uri)
-    const span = (range: Range): SourceRange => ({ startIndex: doc.offsetAt(range.start), endIndex: doc.offsetAt(range.end) })
-    const node = (symbol: DocumentSymbol): SymbolNode => ({
-      name: symbol.name,
-      range: span(symbol.range),
-      selection: span(symbol.selectionRange),
-      children: (symbol.children ?? []).map(node),
-    })
-    return Array.isArray(symbols) ? symbols.map(node) : []
-  }
-
   /**
    * 注释里不带路径的引用（`[Name]`、`[Class.member]`）指到哪：本文件定义的优先，再是导入进来的（见 symbolLinks.ts）。
    * 文档符号同仓库链接找符号那一套（symbols，一批引用一起问时共用一份），导入的名字问语言扩展它的定义（同跳到定义），定义几处取第一处。
    */
-  private async resolveSymbolLink(symbol: string, symbols: (uri: string) => Promise<SymbolNode[]> = uri => this.symbolNodes(uri)): Promise<LinkTarget | undefined> {
+  private async resolveSymbolLink(symbol: string, symbols: (uri: string) => Promise<SymbolNode[]> = documentSymbolNodes): Promise<LinkTarget | undefined> {
     const { document } = this
     try {
       return await resolveReference(symbol, this.imports, {