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.

docs/adr/0059-dev-workflow.md

@@ -74,7 +74,10 @@ directory is how work gets clobbered — worktrees make that impossible.
 Helpers:
 
 - **`scripts/wt-new.sh <branch>`** — fetch `main`, create the branch off
-  `origin/main`, and add the sibling worktree; prints its path.
+  `origin/main` (with `--no-track`, so it doesn't inherit `origin/main` as
+  upstream — otherwise a bare `git push` errors on the name mismatch and
+  suggests the dangerous `HEAD:main`), and add the sibling worktree; prints
+  its path. First push is `git push -u origin <branch>`.
 - **`scripts/wt-rm.sh <branch>`** — remove the worktree for the **named**
   branch, and nothing else. It does **not** scan for "merged" worktrees to
   auto-remove: long-lived branches (`website`, the `ci` line) are merged