tsk-41: document the opt-in nature of plain-git sync
Chose option A from the design discussion: don't auto-mutate the
user's git config, just document the opt-in path more clearly. The
trade-offs that ruled out auto-setup:
- Setting `remote.<name>.push` overrides `push.default`. Once we
configure it, plain `git push` no longer pushes branches the way
the user expects — it pushes whatever's in the configured refspec.
Adding `HEAD` softens but doesn't eliminate the surprise (anyone
on push.default=matching is silently narrowed).
- Mutating `.git/config` from `tsk show` / `tsk list` is invasive.
Some users may not want tsk refs riding along on their pushes.
So `tsk git-setup` stays explicit. AGENTS.md gains a note documenting
the trade-off so users aren't surprised when running it changes their
push behavior; the recommended path for users who don't want that is
to keep using `tsk git-push` / `tsk git-pull` and skip git-setup.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>