using-git-worktrees: the native tool targets the session's repo, and the baseline run assumes exclusive ports

Skill: using-git-worktrees (Step 1a, Step 3). Checked against 6.4.2 and 7.0.0.

• I searched existing issues and PRs and this has not been proposed before (adjacent: #2430 and #2520 cover processes left running after a reported DONE; this is about the worktree skill itself).

What problem does this solve?

1. Native tool vs target repo (Step 1a). The skill says to prefer the native worktree tool and calls git worktree add the "#1 mistake". The native tool creates the worktree in the session's repository. The work was in a sibling repo, at a path the user fixed. The skill does not say the git fallback is the right path when the native tool cannot reach the target repo.
2. Baseline with shared resources (Step 3). The baseline test run starts servers on fixed ports, and another session was running the same suite. Running it would have killed one of the two runs with EADDRINUSE. Isolating files with a worktree does not isolate process resources.

Proposed solution

• Step 1a, one line: "The native tool covers the session's repo. If the target repo is another one (a sibling repo, a path the user fixed), use the git fallback there and say why."
• Step 3: before the baseline, check whether the suite uses shared resources (fixed ports, a database, a common temp dir) that another session or agent may hold. If so, confirm they are free, or defer the baseline to the start of implementation and record the deferral.

Principle: a setup step that runs something with effects outside its own directory has to ask who else uses it right now.