Worktree Isolation

Ensure the current work happens in an isolated workspace, without disturbing the user's main checkout. Most coding harnesses now create a worktree by default at session start, so the common case is that isolation already exists — detect that first and do not create a redundant one.

Order of operations: detect existing isolation -> prefer a native worktree tool -> fall back to plain git. Never create a worktree the harness cannot see.

Step 0: Detect existing isolation

Before creating anything, check whether the current directory is already a linked worktree. Compare the resolved absolute git dir against the resolved absolute common git dir — resolve each to an absolute path first and compare those, not the raw git rev-parse output. Git mixes absolute and relative forms depending on the current directory (from a subdirectory of a normal checkout, --git-dir comes back absolute while --git-common-dir may be relative), so a raw string compare yields a false "already isolated":

git rev-parse --absolute-git-dir                     # absolute git dir for this worktree
(cd "$(git rev-parse --git-common-dir)" && pwd -P)   # absolute shared (common) git dir

If the two absolute paths are equal, this is a normal checkout — continue to Step 1.

If they differ, you are in a linked worktree or a submodule. Distinguish them: