fix(fleet): create the staging worktree before the recut can use it

[swackhamer]

Problem

poll_once ran its staleness re-cut check before ensure_staging_worktree. recut_squad_branch's first act is checkout_detached(staging_path, ...), which hands staging_path to subprocess as cwd — so on a host where ~/.oxidex/worktrees/squad-staging/<squad> doesn't exist yet (a fresh machine, or after stop_parallel_fix.py reaped the worktrees), the recut raises FileNotFoundError before the one function that would have created that worktree ever runs.

Measured 2026-08-04 launching a 20-worker fleet: all 14 mergers died within one second. Every squad/<name> branch was stale against a main that had moved 249 commits, so should_recut fired for all of them simultaneously and the whole merger tier went down together — the configuration where workers keep producing fixes with no publish path at all.

File "scripts/squad_merge_loop.py", line 1565, in poll_once
    recut_result = recut_squad_branch(
File "scripts/squad_merge_loop.py", line 1497, in recut_squad_branch
    checkout_detached(staging_path, origin_ref)
FileNotFoundError: [Errno 2] No such file or directory:
    PosixPath('/Users/allen/.oxidex/worktrees/squad-staging/tail')

Fix

Hoist ensure_staging_worktree above the recut check. It is idempotent — reuse-in-place on an existing directory (reset --hard + clean -fd + checkout), worktree add otherwise — so the move costs one cheap reset/checkout on the common path and makes the recut survive a cold start.

Why it went undetected

Every pre-existing poll_once test passes check_recut=False, so the crashing path was never exercised. RecutSquadBranchTests calls recut_squad_branch directly with a staging worktree already built.

The new test drives poll_once with check_recut=True against an absent staging directory and reproduced the production traceback line-for-line before the fix.

Verification

• New test fails with the exact production FileNotFoundError before the change, passes after
• python -m unittest test_squad_merge_loop — 116/116 pass
• Full just pre-commit gate (fmt-check, cbindgen-check, lint, test) green

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