Make the tagged-release path work on Gitea (closes #65)

No tag could be cut at all: .goreleaser.yaml had no gitea_urls block, so
goreleaser defaulted to the GitHub API, and the repo has zero tags.

.goreleaser.yaml now points at git.eeqj.de. Version derives from git via
a new script/version - exact tag with any leading v stripped, else
dev-<12-char sha>, with a -dirty suffix when tracked files are modified -
replacing the hardcoded 1.0.0-rc.1 that every local build was stamping
regardless of git state. A tag-triggered .gitea/workflows/release.yml
runs goreleaser with a scoped token (RELEASE_TOKEN); script/bootstrap
installs a sha256-verified goreleaser, and make release / release-snapshot
become script shims like every other target.

Two fabrications were removed rather than merely replaced. goreleaser's
snapshot.version_template was `{{ incpatch .Version }}-next`, which
invents a release number from the last tag - and with no tags, from
goreleaser's own fabricated v0.0.0. And internal/cli/version.go gated its
development-build notice on Version == "dev" exactly, so the moment
untagged builds carried a sha that notice would have gone silent and an
unreleased binary would have read as a release. Replaced with a tested
IsDevVersion predicate, and closed at both layers: the Makefile now
refuses to build when script/version yields nothing, and an empty version
counts as a development build - reachable today via
`docker build --build-arg VERSION=`.

The release workflow installs Go from a sha-pinned actions/setup-go
(v5.6.0) using go-version-file, so the compiler that produces released
binaries is pinned like every other external reference. Without it the
first tag push would either fail at goreleaser's before-hook or compile
the published artifacts with whatever unpinned Go the runner happened to
carry - the one unpinned thing in a release path that already refuses an
unpinned goreleaser.

Known gap: the Go tarball setup-go fetches is version-pinned but not
checksum-verified against a value in this repo, unlike the goreleaser
install and the Dockerfile digest.

internal/globals/globals_test.go

@@ -32,3 +32,56 @@ func TestGlobalsNew(t *testing.T) {
 		t.Error("Commit should not be empty")
 	}
 }
+
+// TestIsDevVersion covers the boundary that matters: everything
+// script/version and goreleaser's snapshot template can emit for an
+// untagged build must be recognised as a development build, and a real
+// tag must not be. A plain equality check against "dev" used to decide
+// this, which classified every commit-stamped dev build as a release.
+func TestIsDevVersion(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		version string
+		want    bool
+	}{
+		// What an untagged build produces.
+		{"dev", true},
+		{"dev-b6e4a218a39e", true},
+		{"dev-b6e4a218a39e-dirty", true},
+		// What a tagged build produces (script/version strips the
+		// leading "v", matching goreleaser's .Version).
+		{"1.0.0", false},
+		{"0.1.0", false},
+		{"1.0.0-rc.1", false},
+		{"v1.0.0", false},
+		// A release must not be mistaken for a dev build just because
+		// the string happens to contain "dev".
+		{"1.0.0-dev", false},
+		{"developer", false},
+		// A binary with no version string at all did not get stamped,
+		// which is a build failure, not a release. It must never print
+		// as one. The Makefile refuses to build when script/version
+		// yields nothing; this covers a binary linked some other way.
+		{"", true},
+	}
+
+	for _, tc := range cases {
+		if got := globals.IsDevVersion(tc.version); got != tc.want {
+			t.Errorf("IsDevVersion(%q) = %v, want %v", tc.version, got, tc.want)
+		}
+	}
+}
+
+// TestDefaultVersionIsDev pins the linker-flag contract: an unstamped
+// binary (no -ldflags at all, which is what `go build ./...` and `go
+// install` produce) must report itself as a development build rather
+// than as some default release number.
+func TestDefaultVersionIsDev(t *testing.T) {
+	t.Parallel()
+
+	if !globals.IsDevVersion(globals.DevVersion) {
+		t.Errorf("DevVersion %q is not recognised as a dev version",
+			globals.DevVersion)
+	}
+}