[OL-01] Extract bounded read-only worktree I/O executor

[jackieismpc]

Summary

• Extract the bounded read-only worktree I/O executor into internal/worktree_io.
• Reuse it from status_io_worker while preserving status behavior and fail-closed boundaries.
• Mark OL-01 complete according to the focused-test scope.

Tests

• LIBRA_SKIP_WEB_BUILD=1 ... cargo test --lib internal::worktree_io (21 passed)
• LIBRA_SKIP_WEB_BUILD=1 ... cargo test --lib status_io_worker (14 passed)

Closes #452