refactor: move the transport onto the official Gitea SDK

The transport was hand-rolled net/http against the REST API. The payload shapes
were ours, in internal/wire, which meant every field Gitea learned was a field
somebody here had to notice; and "does this instance have issue dependencies?"
had to be guessed from a status code, because a 404 from a missing route and a
404 from a missing issue look alike.

The SDK settles both. The shapes are maintained by the people who maintain the
server, and the client negotiates the server's version when it is built, so the
dependency endpoint is now gated on `>= 1.20.0` — verified against the release
where the route appears, not assumed. Below the gate nothing is requested at all.

internal/wire keeps what the SDK has no answer for: addressing. The SDK takes an
owner, a name and an int64 and never parses, while `42`, `#42`, `owner/repo#42`
and an issue URL are four spellings of one address, all four are what somebody
has in hand, and Key is what the ledger is keyed by. The payload structs go.

Four things that had to survive the move, and did:

- request bodies still land in .kettle/payload/, now via an http.RoundTripper on
  the client the SDK is given — which is better than before, because it files
  every request rather than the ones a call site remembered to name;
- errors still carry the HTTP status AND the response body, and a decode failure
  on a 2xx is deliberately not an APIError, so the dependency probe cannot read
  a bad decode as "feature missing";
- the number -> slug ledger is untouched, entries still outlive the files they
  name;
- Client.For(repo) still re-points at another repository for one call.

What it cost, written down in AGENTS.md where it happened. internal/mapping's
layering test was a fact about the import graph — nothing in its closure could
open a socket — and the SDK ships its types and its client in one package, so
the test now asserts what is still true: the bridge performs no I/O, checked on
direct imports plus a grep for time.Now. A run makes one extra request before it
does anything. Gitea's issue edit endpoint carries no labels, so a push whose
labels changed needs a second call; push makes it and says so. go.mod requires
go 1.26, which the SDK sets and which is now the floor for building this binary.

internal/issue and internal/project are byte-identical. The domain did not
notice, which is the whole argument for the layering.

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

cli/vendor/github.com/davidmz/go-pageant/LICENSE.txt

@@ -0,0 +1,16 @@
+Copyright (c) 2014 David Mzareulyan
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software
+and associated documentation files (the "Software"), to deal in the Software without restriction,
+including without limitation the rights to use, copy, modify, merge, publish, distribute,
+sublicense, and/or sell copies of the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial
+portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING
+BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.