fix(workspace): honour the startup directory on cold start

[azzliang6]

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Result / 用户结果

Addresses the cold-start part of #479. When no tab is restored at startup (empty session, restore_session=0, missing session.json, tripped boot breaker, or a failed post-update restore), the first terminal now opens in the configured Startup directory instead of the process working directory. With the login-autostart shortcut that process directory is %USERPROFILE%, so the setting was ignored on every boot while Ctrl+T tabs honoured it.

启动时没有恢复出任何标签（空会话、关闭「启动时恢复上次标签」、缺少 session.json、断路器跳闸、更新后恢复失败）时，首个终端现在进入设置里的「启动目录」，不再落在进程启动目录。开机自启动快捷方式的起始位置是 %USERPROFILE%，此前每次开机都会忽略该设置，而 Ctrl+T 新标签是正确的。

Design / 设计边界

• Responsibility and affected modules: gpui_shell/workspace.rs — the RestoreOrDefault and RestoreUpdate cold-start fallbacks call Self::cold_start_cwd() instead of std::env::current_dir(). The helper lives in gpui_shell/workspace/session_recovery.rs (cold restoration) and reuses the existing NebulaWorkspace::startup_directory(), the same authority add_terminal uses, so no second reading of the setting is added.
• Why this belongs here; interfaces that remain unchanged: priority stays "explicit CLI dir > restored session > startup directory > process cwd", matching the legacy shell's preferred_initial_cwd. windowing::initial_startup dispatch is unchanged, so an explicit CLI path still wins and a restored session still keeps its saved cwd.
• Dependency, data-format, threading, or lifetime changes (ADR if applicable): none.
• Compatibility and migration/fallback behavior: when the startup directory is unset or no longer a directory, behaviour is identical to before (inherit the launch directory, the 1.8.0 contract). The second-launch handoff to a resident instance mentioned at the end of [Bug] 设置启动目录后，开机自启动的首个终端仍打开在 %USERPROFILE% #479 is intentionally not changed here.

Evidence / 验证依据

• Commands and actual results (include unrun checks), on Linux x86_64 with the pinned 1.97.1 toolchain:
  • cargo test --locked -p nebula --bin pebrel --features gpui-shell cold_start_tests — 2 passed (and configured_startup_directory_wins_over_the_launch_directory fails when the helper is reverted to the old "launch directory only" behaviour)
  • cargo check --locked -p nebula --bin pebrel --features gpui-shell --tests — passes (only pre-existing warnings)
  • python3 scripts/check_architecture.py --base origin/main (base b6e7b78d) — passes
  • python3 -m unittest scripts.tests.test_architecture_budgets scripts.tests.test_architecture_dependencies scripts.tests.test_architecture_governance — 54 tests OK
  • cargo fmt --all -- --check — passes
  • Not run: Windows build or manual autostart reproduction (no Windows machine available), macOS, the GPUI window tests behind gpui-test-support, the full native suite (left to CI).
• Regression tests: configured_startup_directory_wins_over_the_launch_directory pins that a configured startup directory beats the launch directory (%USERPROFILE% in the autostart case). The old fallback passed std::env::current_dir() straight through, which is exactly the case this test rejects. unset_startup_directory_keeps_inheriting_the_launch_directory covers the preserved unset/invalid path.
• UI changes: none.
• Hot-path changes: none (one settings read on the cold-start path only, as Ctrl+T already does).

Required Review / 必须确认

• I followed CONTRIBUTING.md, docs/architecture.md, and docs/project-constraints.md.
• I split responsibilities, not arbitrary line ranges; no duplicate behavior authority was added.
• python3 scripts/check_architecture.py --base <PR-base-commit> passes; budgets were not inflated to fit the change.
• Tests cover success and failure; platform/feature coverage limitations are stated.
• New messages use typed i18n IDs and matching placeholders; untranslated content has an explicit fallback. (N/A: no new messages)
• Governance changes include a counterexample, corrected contract, tests, and a maintainer-reviewed decision. (N/A)

Checkboxes explain the review; they do not replace CI or maintainer approval.

[Kuddev]

已审阅两处冷启动入口及 startup_directory / initial_startup / restore 路径。修复复用现有目录校验，只在恢复未产生标签时读取配置目录；不改变显式 CLI 目录、已恢复会话目录及未配置/无效目录的回退。没有新依赖、线程或常驻缓存，成本限于冷启动路径。

维护者已直接把测试目录改为中性名称并同步 main。本地 Windows 产品测试构建使用 gpui-test-support，cold_start_tests 两项通过；架构、名称范围与差异检查通过。未运行真实登录自启动或全套本地 UI 场景。批准精确 head 的代码；合并仍等待该 head 的必需 CI，旧 head 的成功记录不替代它。

[Kuddev]

已同步 main 中合入的 #351。复核最终 diff 仍仅为原来的两处冷启动回退及共享 helper/测试，没有覆盖 WSL 主线逻辑。最终 head aedfd3e 的 Windows 产品 cold_start_tests 再次 2/2 通过，架构、格式、名称范围与差异检查通过。维持代码批准，等待这一 head 的必需 CI 完成后合并。