feat(routing): create GitHub destination repo before configuring push-mirror

Gitea's push-mirror cannot push to a non-existent remote — it just
runs 'git push' against whatever URL it's given. So a project_create
flow that only configures the mirror leaves the GitHub side as an
unfulfillable URL.

New internal/githubclient package: single-purpose client that POSTs
/user/repos to create an empty private repo (auto_init=false so the
first mirror push doesn't conflict with a generated README). Treats
422 'name already exists' as idempotent success via ErrAlreadyExists.
401/403 are surfaced as 'PAT missing repo scope or invalid' so the
operator sees the real cause instead of a vague upstream error.

Skill wiring:
- New stepCreateGitHub between stepCreateRepo and stepMirror in the
  orchestrator.
- Skipped entirely when Config.GitHub is nil (degraded mode — the
  routing pod runs without GITHUB_PAT, mirror config still lands,
  but the actual sync to github fails until the repo exists).
- cmd/routing/main.go constructs githubclient.New(GitHubPAT) only
  when the PAT is set; the skill receives nil otherwise.

Tests:
- happy path: fake github 201 + assertions that the 'reached' array
  is [create_repo, create_github_repo, mirror, infra_commit, issue].
- github 422 already-exists: idempotent, all gitea steps still run.
- github 401: returns failed_step=create_github_repo, no mirror or
  later steps.
- degraded mode (Config.GitHub nil): reached omits create_github_repo,
  rest of the flow runs unchanged.

Updated existing tests to read [skill, gh] from newSkill instead of
just skill, and adjusted reached-array expectations to include the
new step.

Tracks #10.

internal/githubclient/client_test.go

@@ -0,0 +1,71 @@
+package githubclient_test
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/mathiasbq/supervisor/internal/githubclient"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCreateRepo_Success(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, http.MethodPost, r.Method)
+		assert.Equal(t, "/user/repos", r.URL.Path)
+		assert.Equal(t, "token ghp_test", r.Header.Get("Authorization"))
+		var args map[string]any
+		b, _ := io.ReadAll(r.Body)
+		_ = json.Unmarshal(b, &args)
+		assert.Equal(t, "test-repo", args["name"])
+		assert.Equal(t, true, args["private"])
+		assert.Equal(t, false, args["auto_init"])
+		w.WriteHeader(http.StatusCreated)
+		_, _ = w.Write([]byte(`{"full_name":"mathiasb/test-repo","html_url":"https://github.com/mathiasb/test-repo","clone_url":"https://github.com/mathiasb/test-repo.git","private":true}`))
+	}))
+	defer srv.Close()
+
+	c := githubclient.New("ghp_test").WithBaseURL(srv.URL)
+	r, err := c.CreateRepo(context.Background(), "test-repo", "desc", true)
+	require.NoError(t, err)
+	assert.Equal(t, "mathiasb/test-repo", r.FullName)
+	assert.True(t, r.Private)
+}
+
+func TestCreateRepo_AlreadyExists(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusUnprocessableEntity)
+		_, _ = w.Write([]byte(`{"message":"Validation Failed","errors":[{"resource":"Repository","code":"custom","field":"name","message":"name already exists on this account"}]}`))
+	}))
+	defer srv.Close()
+
+	c := githubclient.New("ghp_test").WithBaseURL(srv.URL)
+	_, err := c.CreateRepo(context.Background(), "x", "", false)
+	require.Error(t, err)
+	assert.True(t, errors.Is(err, githubclient.ErrAlreadyExists))
+}
+
+func TestCreateRepo_Unauthorized(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusUnauthorized)
+		_, _ = w.Write([]byte(`{"message":"Bad credentials"}`))
+	}))
+	defer srv.Close()
+
+	c := githubclient.New("ghp_test").WithBaseURL(srv.URL)
+	_, err := c.CreateRepo(context.Background(), "x", "", false)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "PAT missing repo scope")
+}
+
+func TestCreateRepo_NoToken(t *testing.T) {
+	c := githubclient.New("")
+	_, err := c.CreateRepo(context.Background(), "x", "", false)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "github pat not configured")
+}