feat(sidebar): keep multi-root project headers project-only

[im-ian]

Summary

Multi-root project headers now delegate root-scoped creation to the individual root rows and keep only project-level actions at the group level.

1. Header scoping — hide direct regular and worktree session buttons whenever a project has an extra source root.
2. Project-only menus — make both the ellipsis and right-click menus expose source-folder management, project settings, Finder/path actions, and project close only.
3. Accessible localization — give the multi-root ellipsis a dedicated project-menu label in all supported languages.
4. Regression coverage — verify multi-root headers contain no creation actions while single-root headers keep their existing shortcuts.

Expected impact

Area	Before	After
Multi-root project header	Creation buttons and mixed workspace/session menu target the primary root implicitly	No root-scoped creation; project management actions only
Source-root rows	Per-root session and workspace creation	Unchanged and remains the explicit creation surface
Single-root project header	Direct session buttons and creation menu	Unchanged

Design notes

• A project is treated as multi-root when source_paths contains at least one extra root, meaning the primary root plus one or more source roots.
• The ellipsis and right-click menu share the same project-management item list in multi-root mode so alternate interaction paths cannot expose different scopes.
• Session creation plumbing is unchanged; this only removes ambiguous entry points from the group header.

Test plan

• pnpm run typecheck — passes
• pnpm exec playwright test tests/e2e/sidebar.spec.ts --grep "multi-root project header|project header exposes regular and worktree session buttons" — 2 tests pass
• pnpm run test — 109 files and 1,287 tests pass
• pnpm exec playwright test tests/e2e/sidebar.spec.ts — 52 tests pass
• git diff --check — passes