feat(atlas): surface the user's own Gitea repos on stage 05 (#10)

Adds the repos panel from #4's "Not yet shipped" list, following the shipped
issues slice (b2761a4) structurally rather than inventing a second approach:

- internal/atlas/repos.go — RepoNodes(), a pure parser over an already-decoded
  /user/repos response. Archived repos are dropped: the atlas shows where work
  happens, and an archived repo is by definition somewhere it does not.
- internal/gitea/client.go — MyRepos(), same PAT and same contract as MyIssues:
  errors when GITEA_TOKEN is unset so the caller skips the overlay rather than
  issuing an unauthenticated request. That unset path is the one running in
  prod today (#4), so it is tested, not assumed.
- internal/web/handler.go — wired into liveOverlay().

Stage 05 is the right home — repos are what the execution stage acts on — but
#11 had just claimed its `generate` key for stuck-badges, and a stage could
only name one source. So `generate` now accepts either spelling:

    "generate": "ci-jobs"
    "generate": ["stuck-badges", "gitea-repos"]

The bare string stays valid and marshals back as a bare string, so no authored
stage is rewritten and the frontend contract is unchanged. Stages 04 and 08
already share stuck-badges between them; this is the same growth one level in.

The overlay merge also moves out of the handler into atlas.PrependGenerated,
which is unit-testable where the inline loop was not — it sat behind a network
call. It covers the issues path too, and fixes a latent aliasing bug there: the
old `append(liveNodes, authored...)` wrote into the live slice's backing array,
so two stages sharing a key would corrupt each other. That shape is no longer
hypothetical now that stuck-badges spans three stages.

Verified end-to-end both ways: without GITEA_TOKEN the atlas returns 200 with
stage 05's authored nodes intact; with a real token stage 05 serves both
overlays at once — stuck badges, then eight live repos, then the authored
nodes — and a repo with no language emits one tag rather than an empty pill.

task check green (golangci-lint 0 issues, vet, -race).

Closes #10.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/atlas/overlay_test.go

@@ -0,0 +1,66 @@
+package atlas_test
+
+import (
+	"testing"
+
+	"git.d-ma.be/mathias/cad-atlas/internal/atlas"
+)
+
+func stagedAtlas() *atlas.Atlas {
+	return &atlas.Atlas{Stages: []atlas.Stage{
+		{No: "STAGE 03", Generate: atlas.GenerateKeys{"gitea-issues"}, Nodes: []atlas.Node{{Title: "authored issue node"}}},
+		{No: "STAGE 05", Generate: atlas.GenerateKeys{"gitea-repos"}, Nodes: []atlas.Node{{Title: "authored repo node"}}},
+		{No: "STAGE 08", Nodes: []atlas.Node{{Title: "untouched"}}},
+	}}
+}
+
+func TestPrependGenerated_LiveNodesComeFirstAndAuthoredSurvive(t *testing.T) {
+	a := stagedAtlas()
+
+	atlas.PrependGenerated(a, "gitea-repos", []atlas.Node{{Title: "live repo"}})
+
+	got := a.Stages[1].Nodes
+	if len(got) != 2 {
+		t.Fatalf("want 2 nodes, got %d: %+v", len(got), got)
+	}
+	if got[0].Title != "live repo" || got[1].Title != "authored repo node" {
+		t.Fatalf("order wrong: %+v", got)
+	}
+}
+
+func TestPrependGenerated_OnlyTouchesMatchingStage(t *testing.T) {
+	a := stagedAtlas()
+
+	atlas.PrependGenerated(a, "gitea-repos", []atlas.Node{{Title: "live repo"}})
+
+	if len(a.Stages[0].Nodes) != 1 || a.Stages[0].Nodes[0].Title != "authored issue node" {
+		t.Fatalf("stage 03 disturbed: %+v", a.Stages[0].Nodes)
+	}
+	if len(a.Stages[2].Nodes) != 1 || a.Stages[2].Nodes[0].Title != "untouched" {
+		t.Fatalf("stage 08 disturbed: %+v", a.Stages[2].Nodes)
+	}
+}
+
+// The graceful-skip contract: with GITEA_TOKEN unset the overlay yields no
+// nodes, and the atlas must still render its authored placeholders unchanged.
+func TestPrependGenerated_NoLiveNodesLeavesAuthoredIntact(t *testing.T) {
+	a := stagedAtlas()
+
+	atlas.PrependGenerated(a, "gitea-repos", nil)
+
+	if len(a.Stages[1].Nodes) != 1 || a.Stages[1].Nodes[0].Title != "authored repo node" {
+		t.Fatalf("authored nodes lost on empty overlay: %+v", a.Stages[1].Nodes)
+	}
+}
+
+func TestPrependGenerated_UnknownKeyIsANoop(t *testing.T) {
+	a := stagedAtlas()
+
+	atlas.PrependGenerated(a, "no-such-key", []atlas.Node{{Title: "live"}})
+
+	for i, s := range a.Stages {
+		if len(s.Nodes) != 1 {
+			t.Fatalf("stage %d changed: %+v", i, s.Nodes)
+		}
+	}
+}