feat(workspace): open project folders in new terminal tabs

[Aschenbath]

Result / 用户结果

Fixes #124.

• Add Open project folder… to the upper-left toolbar in both sidebar and top-tab layouts.
• The native single-directory picker opens a new terminal tab in the selected folder using the configured default shell.
• Existing terminals receive no commands. A saved profile's startup directory is overridden only in the new tab's launch snapshot; the saved profile itself is unchanged.
• Canceling is a no-op. Invalid or unavailable folders show a localized warning and can be retried.

Design / 设计边界

• Folder selection, validation and launch adaptation live in a focused workspace module. Existing tab/session creation remains the authority for terminal startup.
• Use GPUI's existing path picker; filesystem validation runs on the background executor. A workspace-owned task prevents duplicate pickers and ends with its workspace.
• Reuse the titlebar's drag-safe control wrapper. The entry has a 32×32 logical-pixel hit box, an accessible label, a visible focus outline, and local Tab/Enter/Space access; no global shortcut is added.
• Account for the entry in the top-tab strip's width budget. Move the existing titlebar-control helper into the titlebar module without changing its behavior; no architecture budget is raised.
• No new dependency, persistent setting, alternate terminal-launch implementation, or shell-command interpolation.

Evidence / 验证依据

Local checks on this branch:

Check	Actual result
cargo +1.97.1 test --offline --locked -p nebula --bin pebrel --features gpui-test-support -- --test-threads=1	1396 passed, 0 failed, 10 ignored
cargo +1.97.1 check --offline --locked -p nebula --bin pebrel --features gpui-shell --tests	Passed
cargo +1.97.1 test --offline --locked --manifest-path tools/i18n-contract/Cargo.toml	14 passed, 0 failed, 1 ignored
python -B scripts/check_architecture.py --base ce068600b2382d71301070535dcdb4e038156b56	Passed; 576 source files, 9 crates
python -B -m unittest scripts.tests.test_architecture_budgets scripts.tests.test_architecture_dependencies scripts.tests.test_architecture_governance	51 passed
Changed Rust files: rustfmt --edition 2024 --config skip_children=true --check; git diff --check	Passed

Eight focused regressions cover both actual toolbar layouts, full padded hit area, duplicate clicks, the single-directory picker contract, cancel/retry, real Tab/Enter routing, invalid/file selections, picker failure, workspace closure, failed shell startup, profile snapshot isolation, and Unicode/space-containing paths.

The opt-in Windows native acceptance test was also run successfully in Nord/sidebar (1080 logical pixels wide) and MintLight/top tabs (760 logical pixels wide) at 192 DPI / 200% scaling. It creates isolated test terminals, checks the new shell's actual working directory, and verifies that the original tab/entity and cwd remain unchanged. Screenshots below show the real toolbar's keyboard focus in both layouts; fixture paths and terminal contents are excluded.

Limits: the native acceptance test injects the selected directory at the picker-result boundary so it does not open a modal dialog over other work. The picker/button/cancel/keyboard route is exercised separately with GPUI test support; manual Windows picker interaction, Linux/macOS native behavior, WSL/UNC directories and other DPI values are not verified here. Full-workspace cargo fmt --all -- --check still reports unrelated existing formatting differences; only changed Rust files were formatted. These are local results, not a claim about CI or a released build.

No render hot loop is changed; filesystem work only occurs after an explicit folder selection. The installed application was not replaced or restarted for this PR.

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.
• Governance changes include a counterexample, corrected contract, tests, and a maintainer-reviewed decision. N/A: no governance changes.