✨ feat(vscode): 新增第五种工单会话「原型会话」

照测试会话那条链路补一份 prototype：

- 消息与路由：session/start-prototype、session/resume-prototype、
  issue/update-prototype-profile-path，都进只读门禁
- 会话：handleStartPrototypeSession / handleResumePrototypeSession，
  终端名 issue-N-原型；cwd 有 worktree 用 worktree、没有退主仓库根
  （原 resolveTestSessionCwd 改名 resolveWorktreeOrRootCwd 两种会话共用）
- 不要求有 PR，任何阶段都能起；前置工单锁仍然拦
- 提示词：新增 prompts/prototype.md，设置里可改，占位符 {issueNumber}
- profile：工单级 prototypeProfilePath > 设置里的原型默认 profile >
  内置默认，与其它会话互不回退
- 状态字段 prototypeSessionId / prototypeProfilePath / prototypeTabOpen
  贯通 gitea、youtrack、本机 state 与移交包
- 详情面板加「原型会话id」「原型配置文件」两行，设置面板加对应两项

Claude-Session: https://claude.ai/code/session_01Kgmb1LvrPmFX3NoUDB4zZC

vscode/src/youtrack/issueLoader.ts

@@ -68,9 +68,11 @@ function toIssue(baseUrl: string, it: YouTrackIssue): Issue {
     implementSessionId: str(state.implementSessionId),
     reviewSessionId: str(state.reviewSessionId),
     testSessionId: str(state.testSessionId),
+    prototypeSessionId: str(state.prototypeSessionId),
     profilePath: str(state.profilePath),
     brainstormProfilePath: str(state.brainstormProfilePath),
     testProfilePath: str(state.testProfilePath),
+    prototypeProfilePath: str(state.prototypeProfilePath),
     specFile: str(state.specFile),
     planFile: str(state.planFile),
     prDiffFile: str(state.prDiffFile),

vscode/src/youtrack/stateComment.ts

@@ -20,7 +20,7 @@ import { addComment, listComments } from './api'
 export const STATE_MARKER = '<!--spx-state-->'
 
 /** Same field set as the Gitea state blob — kept independent to avoid coupling. */
-const KNOWN_STATE_FIELDS = ['column', 'sessionId', 'implementSessionId', 'reviewSessionId', 'testSessionId', 'profilePath', 'brainstormProfilePath', 'testProfilePath', 'specFile', 'planFile', 'prDiffFile', 'pr', 'prMerged', 'prMergedAt', 'branch', 'worktreePath', 'implementStatus', 'color', 'autoReview', 'handoffAttachmentId', 'handoffFrom'] as const
+const KNOWN_STATE_FIELDS = ['column', 'sessionId', 'implementSessionId', 'reviewSessionId', 'testSessionId', 'prototypeSessionId', 'profilePath', 'brainstormProfilePath', 'testProfilePath', 'prototypeProfilePath', 'specFile', 'planFile', 'prDiffFile', 'pr', 'prMerged', 'prMergedAt', 'branch', 'worktreePath', 'implementStatus', 'color', 'autoReview', 'handoffAttachmentId', 'handoffFrom'] as const
 
 /**
  * Parse a comment body into a state object, or null if it isn't a state