fix: keep request payloads out of the issue store

`labels.py` handed `_gitea.api` the issue store as a place to put the
request file, and on a checkout without a store that quietly created
`tmp/issues/.payload/`. Bootstrapping a repository's labels touches no
issue at all, so the one rule the store has — nothing materializes it as
a side effect of a write — was broken by an operation that has no
business knowing the store exists.

Where a request body goes was never the caller's decision to make. It is
now the transport's: `tmp/payload/`, resolved from `_gitea.py`'s own
location the way both domains resolve theirs, so every caller — sync and
wiki alike — writes to one directory whatever it was invoked from, and
`out_root` is gone from `api`, `add_dependency` and all six call sites.
The directory is created by the first write of a run and not before: a
`--dry-run` leaves nothing behind. `tmp/` is already gitignored.

The name carries the distinction the old path lost. A store holds the
only copy of something; this holds debris kept for a retry or a
post-mortem, and deleting it costs nothing. A dotdir sitting among an
issue's files claimed otherwise, and `ls tmp/issues` started lying about
what existed.

tests/test_payload_root.py runs the real `labels.py` in a throwaway repo
against a fake `tea` on PATH: no store appears, the payloads land in
tmp/payload/, a dry run writes nothing, and a run from a subdirectory
still resolves to the repo root. Two source checks keep the callers from
drifting apart again.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/test_drop_after_push.py

@@ -124,7 +124,7 @@ class FakeTracker(object):
     # -- the seam ----------------------------------------------------------
 
     def api(self, login, endpoint, method="GET", payload=None,
-            payload_name=None, out_root=None, allow_fail=False):
+            payload_name=None, allow_fail=False):
         self.calls.append((method, endpoint, payload))
         path = endpoint.split("?")[0]