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/internal/mapping/mapping.go

@@ -1,5 +1,5 @@
-// Package mapping is md <-> Gitea JSON. The whole translation, and only the
-// translation.
+// Package mapping is md <-> Gitea's payloads. The whole translation, and only
+// the translation.
 //
 // Pure functions: no network, no filesystem, no flags, no clock. Give it a
 // payload and it hands back a domain issue; give it an issue and it hands back
@@ -7,13 +7,21 @@
 // tested without a Gitea anywhere, and it is the one package to open when the
 // two representations disagree.
 //
-// Direction of knowledge: this package imports the domain and the protocol
-// (internal/wire), and nothing imports it but the command layer. The domain
-// never imports it, and TestDomainDependsOnNothing over in internal/issue fails
-// the moment it does; the transport never imports it either, and
+// Direction of knowledge: this package imports the domain and the Gitea SDK,
+// and nothing imports it but the command layer. The domain never imports it,
+// and TestDomainDependsOnNothing over in internal/issue fails the moment it
+// does; the transport never imports it either, and
 // TestTransportDoesNotImportTheDomain over in internal/gitea says so. Both
-// sides speak wire's shapes, which is what lets the two meet without either one
-// reaching into the other.
+// sides speak the SDK's shapes, which is what lets the two meet without either
+// one reaching into the other.
+//
+// WHAT THAT COSTS, SAID OUT LOUD. The shapes used to be internal/wire's, a
+// package that imported the standard library and nothing else, so "the bridge
+// cannot reach the network" was a fact about the import graph. code.gitea.io/
+// sdk/gitea carries an HTTP client, so it is not any more. What is still true
+// is that nothing HERE does I/O, and layering_test.go asserts the version of
+// the rule that can still be checked: no os, no net/http, no transport, no
+// configuration, no clock, and no third party but the SDK.
 //
 // What crosses the boundary, and what does not:
 //