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>

skills/sync/SKILL.md

@@ -298,6 +298,10 @@ decision, not a migration. A color or `exclusive` that drifted is printed, and
 changed only under `--fix`. Running it twice creates nothing. `tech/*` and
 `comp/*` are open-ended by design and stay push-created.
 
+Labels belong to the repository, not to any issue, so this one runs on a
+checkout with no store and leaves it that way — nothing here reads `tmp/issues/`
+and nothing creates it. The request bodies go to `tmp/payload/` (below).
+
 A milestone must already exist in the repo — push attaches, it does not create.
 
 `branch:` is Gitea's `ref`, the branch the work actually lives on. Push fills
@@ -354,6 +358,13 @@ precisely so the mechanism this section rules out is not needed.
 
 ## Rich payloads for everything else
 
+Every body these scripts send is written to `<repo>/tmp/payload/<name>.json`
+first and passed as `-d @file`, then kept for a retry or a look at what actually
+went up. One gitignored directory for all of them, chosen by the transport and
+not by the caller. **It is not a store**: nothing in it is anybody's only copy,
+and it is never `tmp/issues/` — a command that touches no issue must not leave
+an issue store behind.
+
 Comments and issues are wrapped by the scripts above. For **other** entities
 (pulls, releases, PATCHing something these scripts do not cover), entity
 subcommands like `tea pulls create` hang on a large or formatted body — an