verify-worktree: don't accept a stale real dir as a valid link on POSIX

[dsarno]

Problem

script/verify-worktree's Invariant-2 check has a macOS/Linux blind spot. The link test_project/addons/godot_ai should be a symlink into this worktree's plugin/addons/godot_ai (or, on Windows, a directory junction).

The elif [ -d "$link" ] fallback was meant only to accept genuine Windows directory junctions, which appear as plain dirs to bash. But on macOS/Linux it also matched a stale real directory — a full copy of an old plugin left behind by a self-update smoke test or a botched checkout. The script saw plugin.gd inside it, printed [ok], and exited 0 without repairing — so a developer/agent silently tested OUTDATED plugin source. Hit in practice: a stale copy lacked a branch's game_helper.gd fix while [ok] was printed.

Fix

Gate the -d fallback to Windows only, reusing the same ${OS}/${OSTYPE} detection the script already uses for link creation. On POSIX, a non-symlink directory now falls through to the existing rm -rf "$link" + recreate-symlink path. Windows junction support (junctions look like plain dirs to bash, matched via plugin.gd presence) is unchanged.

Tests

New tests/unit/test_verify_worktree_link.py drives the script against a throwaway sandbox repo (the real worktree's link is never touched), POSIX-only:

• stale real dir → repaired to symlink (the regression; also asserts it now resolves to the canonical plugin.gd, not the stale copy)
• healthy symlink left intact ([ok], idempotent)
• missing link created
• wrong-target symlink repaired

All 4 pass; ruff check clean; bash -n syntax-ok.

Manual repro

cd ~/godot-ai
rm -rf test_project/addons/godot_ai
mkdir -p test_project/addons/godot_ai
touch test_project/addons/godot_ai/plugin.gd
script/verify-worktree          # before: [ok] (BUG). after: recreates the symlink.
ls -ld test_project/addons/godot_ai   # confirms it's a symlink again

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR tightens script/verify-worktree so POSIX worktrees no longer accept a stale real directory as a valid test_project/addons/godot_ai plugin link.

Changes:

• Adds Windows-only gating for the directory fallback used to accept junctions.
• Adds POSIX unit coverage for stale directory repair, healthy symlinks, missing links, and wrong-target symlinks.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
script/verify-worktree	Restricts plain-directory acceptance to Windows junction scenarios.
tests/unit/test_verify_worktree_link.py	Adds sandboxed pytest coverage for link creation and repair behavior.

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