fix(workflow): wt-new branches with --no-track (no inherited origin/main upstream)

Branching off origin/main set the new branch's upstream to origin/main, so a
bare `git push` failed with a name-mismatch and suggested the dangerous
`git push origin HEAD:main`. Add --no-track so the branch carries no upstream
until the first `git push -u origin <branch>` sets the right one — keeping the
fresh-from-origin/main base without the tracking side-effect.

Add a test-wt.sh assertion that a new branch has no upstream, and note the
behaviour + first-push form in ADR-0059 / CLAUDE.md.

scripts/wt-new.sh

@@ -47,7 +47,13 @@ log "fetching origin/main"
 git fetch --quiet origin main || die "git fetch origin main failed"
 
 log "creating branch '$branch' + worktree at $target"
-git worktree add --quiet -b "$branch" "$target" origin/main \
+# `--no-track`: branch off origin/main for a fresh base, but DON'T inherit
+# origin/main as the upstream. Without this, branching off a remote-tracking
+# ref makes git set the new branch's upstream to origin/main, so a later bare
+# `git push` fails with a name-mismatch and suggests the dangerous
+# `git push origin HEAD:main`. With no upstream, the first `git push -u origin
+# <branch>` sets the correct one.
+git worktree add --quiet --no-track -b "$branch" "$target" origin/main \
   || die "git worktree add failed"
 
 log "ready — cd into it:"