git reads info/exclude from the common dir for a linked worktree (only
info/sparse-checkout is per-worktree), so the entry written into
<common>/worktrees/<name>/info/exclude was never honoured and CLAUDE.local.md
showed as untracked -- at risk of being swept into a worker's PR. Derive the
common dir (<common>/worktrees/<name> -> <common>) and write there. Found by
dogfooding a real spawn on fleet01; the test now uses the real worktree layout
and asserts the entry lands in the common dir, not the per-worktree gitdir.
Dai Ha
a5efb7c676