feat(sidebar): create worktree sessions from source-folder headers

[im-ian]

Summary

Source folders now expose a one-click worktree-session action directly from their sidebar headers.

1. Direct creation — add a Git branch action to each source root header and route it through the existing scoped isolated-session flow.
2. Accessible localization — add workspace-specific tooltip and ARIA copy in English, Korean, Japanese, and Simplified Chinese.
3. Regression coverage — verify the button targets the source repository and requests an isolated regular session.

Expected impact

Area	Before	After
Source-folder worktree sessions	Open the session menu, then choose the worktree action	Create one directly from the source-folder header
Target repository	Selected through the existing workspace-scoped flow	Explicitly verified to remain the source root
Other workspace rows	Existing header actions	Unchanged

Design notes

• The shortcut appears only for a source-root default workspace row. Project headers already expose the same direct action, while named and worktree workspaces keep their existing menus.
• Creation reuses onAddSession(true, "regular", "terminal"), preserving the established request builder, store assignment, and backend API path.

Test plan

• pnpm run typecheck — passes
• pnpm run test — 109 files and 1,287 tests pass
• pnpm exec playwright test tests/e2e/sidebar.spec.ts --grep "a source folder header creates a worktree session" — 1 test passes
• pnpm exec playwright test tests/e2e/sidebar.spec.ts — 51 tests pass
• git diff --check — passes