fix: clean up OpenCode streams and allow external working directories

Completed OpenCode requests left their event streams open across repeated and
parallel conversations. Each request now closes its own subscription on exit
without cancelling peer sessions, and cancellation preserves its original cause.

Direct-model Execute commands no longer restrict cwd to the workspace. Absolute
paths, parent-relative paths, and symlinks to external directories are accepted.
The workspace remains the default and relative-path base; Ask stays read-only,
and local execution binding, directory validation, and process cleanup remain.

Add real OpenCode lifecycle and real Shell tool-loop regressions, plus matching
feature documentation. Focused checks: 182 passed, 1 skipped; typecheck and lint
passed. Full shared-worktree suite: 3542 passed, 57 skipped, 1 unrelated sidebar
heading failure, reproduced separately and documented.

Release note: 修复 OpenCode 连续及并行对话结束后事件连接未释放的问题；直连模型 Execute 命令现在可直接指定工作区外的绝对路径、相对路径或符号链接目录，Ask 只读边界保持不变。

docs/features/direct-model-agent/user-stories.md

@@ -42,12 +42,13 @@ Shell、目录、退出码、输出和耗时。
 Given 当前项目位于 macOS 或 Linux 本机，When 模型运行同一工具，Then GoodBuddy 使用
 Bash，Bash 不可用时使用 Sh，工具契约保持不变且实际 Shell 明确可见。
 
-### US-A3 使用工作区子目录
+### US-A3 选择命令工作目录
 
 关联：FR-3。
 
-Given 项目包含多个包，When 模型传入工作区内相对目录，Then 命令在该目录执行；When
-路径越出工作区、不是目录或不存在，Then 命令不启动并返回准确错误。
+Given 当前请求为本机 Execute，When 模型传入绝对路径或相对工作区路径，Then 命令在
+解析后的目录执行，包括工作区外目录和符号链接目标，And 后续省略 `cwd` 时仍使用原工作区。
+When 目标不是目录或不存在，Then 命令不启动并返回准确错误。
 
 ### US-A4 根据非零退出继续修正