fix(sidebar): restore worktree action for primary project roots

[im-ian]

Summary

Multi-root project sidebars now expose the missing worktree-session shortcut on the primary repository row.

1. Primary-root action — show the worktree-session button for every default root workspace row instead of limiting it to source roots.
2. Regression coverage — verify that primary and source root buttons each create an isolated regular session against their own repository path.

Expected impact

Sidebar flow	Before	After
Multi-root primary repository row	No direct worktree-session action	Worktree-session button is available on hover
Source repository row	Worktree-session button available	Unchanged
Single-root project header	Worktree-session button available	Unchanged

Design notes

• Multi-root project headers remain limited to project-management actions so the repository target stays explicit at each root row.
• isDefaultProjectFolder identifies both primary and source root rows; named workspaces and worktree workspaces remain excluded from the shortcut.
• Session creation continues through the existing workspace-scoped flow, preserving the selected root repoPath, isolated flag, and regular session kind.

Test plan

• pnpm run typecheck
• pnpm run test — 109 files, 1,290 tests passed
• pnpm run build:frontend
• pnpm exec playwright test tests/e2e/sidebar.spec.ts — 53 tests passed