[gh-8226] Add "Open with <Editor>" item to tab and file-tree menus (flag-gated)

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Summary

Adds an "Open with " item to two existing right-click context menus — session tabs and file-tree directory headers — that launches the user's configured external editor on the relevant directory. The menu label is rendered from the existing code.editor.open_file_editor setting; no editor is hardcoded. Gated behind FeatureFlag::OpenDirectoryInExternalEditor (Cargo feature open_directory_in_external_editor), default off.

Implements specs/gh-8226-open-with-editor/ (companion spec PR #12901). This PR sits on top of #12901; once #12901 merges to master, the diff here will reduce to just the code changes.

Motivation

Issue #8226 asks for a way to open a directory in $EDITOR from Warp. Today's "Open in editor" plumbing supports files only — there is no fast path from "this tab represents a project" or "this is a folder in the file tree" to "open this folder in my editor." Users currently fall back to typing code . / cursor . or alt-tabbing into the editor's recent-projects list; both break flow.

This PR ships one of three complementary UX entry points discussed on #8226 (the other two — click-on-directory-paths in terminal output, and the prompt-area chip prototyped by @ATERCATES — remain out of scope here and are tracked separately).

What's changed

• util::file::external_editor::open_directory_in_external_editor — new entry point. Reads EditorSettings::open_file_editor and dispatches to per-platform implementations.
• Per-platform open_directory — mac.rs uses /usr/bin/open -a <app-bundle> <dir> (the macOS-native form that VSCode / Cursor / Windsurf / Zed / Sublime / Atom / JetBrains all interpret as "open this folder as a workspace"); linux.rs and windows.rs reuse each platform's existing editor-launch path with a None line/column.
• Shared label helper — open_with_editor_menu_label* renders "Open with " (ExternalEditor), "Open in editor" (SystemDefault / EnvEditor with $EDITOR set), or None (Warp viewer, or EnvEditor without $EDITOR).
• Session-tab menu (tab.rs) — new open_in_editor_menu_items section, inserted between modify_tab_menu_items and close_tab_menu_items. Dispatched via WorkspaceAction::OpenTabCwdInExternalEditor, which reads the focused session's current_working_directory().
• File-tree menu (code/file_tree/view.rs) — adds the item directly after "Reveal in Finder" for DirectoryHeader entries only (not files). Dispatched via FileTreeAction::OpenDirectoryInExternalEditor. Threads &AppContext into context_menu_items to read the editor setting.
• Feature flag — OpenDirectoryInExternalEditor registered in crates/warp_features/src/lib.rs and gated via app/Cargo.toml + app/src/features.rs.

Out of scope

• Hijacking clicks on directory paths printed inside terminal output (the original ask in Add option to open directories in $EDITOR #8226).
• Prompt-area "Open in IDE" chip (@ATERCATES's prototype on a fork).
• Warp Drive workspace switcher (drive/index.rs render_workspace_picker) — cloud entities, no local cwd.
• SSH / remote terminal cwd resolution. File-tree remote entries are already excluded via the existing is_remote_item gate; the tab cwd reader silently no-ops when the focused session has no resolvable local cwd.
• Line/column positioning (directories have no caret).

Validation

• Unit tests (external_editor/mod_tests.rs, 4 new): menu_label_external_editor_uses_editor_display_name, menu_label_system_default_falls_back_to_generic_label, menu_label_warp_choice_yields_no_item, menu_label_env_editor_requires_editor_var_to_be_set. All four EditorChoice variants covered.
• cargo check --lib and cargo check --lib --features open_directory_in_external_editor both pass (flag-on and flag-off code paths compile).
• cargo clippy --lib --features open_directory_in_external_editor clean for the new code.
• cargo test --lib --features open_directory_in_external_editor external_editor — all 33 external_editor tests pass (29 existing + 4 new).
• Manual end-to-end on macOS: WarpOss dev build, Settings → Code → External editor → VS Code → right-click tab → "Open with VSCode" → VS Code opens the tab's cwd as a workspace. Same for file-tree directory right-click. With Settings → SystemDefault, label becomes "Open in editor" and clicking falls back to Finder per spec Behavior 4.

Related issues

• Refs Add option to open directories in $EDITOR #8226 (the tab + file-tree right-click UX vector for the broader "open directory in \$EDITOR" ask).
• Depends on [gh-8226] Spec: open dir in external editor from tab + file tree menus #12901 (spec PR). Once [gh-8226] Spec: open dir in external editor from tab + file tree menus #12901 lands, this PR's diff reduces to just the code commits.

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Every PR must be linked to a same-repo issue before Oz can review it.

Next step: open or find a same-repo issue describing this change, then link it to this PR by adding Closes #123 to the PR description (or using the "Development" sidebar on GitHub). A maintainer will mark the issue ready-to-implement when it is ready. Once it is marked, comment /oz-review to re-trigger review.

See the contribution guidelines for the full readiness model.

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