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.

.gitea/workflows/release.yml

@@ -0,0 +1,60 @@
+name: release
+on:
+  push:
+    tags: ["v*"]
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+      # actions/checkout v4, 2024-09-16
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5
+        with:
+          # goreleaser needs the tags and the full history: the version
+          # it stamps comes from the tag, and the changelog comes from
+          # the commits since the previous one. A shallow checkout
+          # silently produces a mislabelled release.
+          fetch-depth: 0
+      # goreleaser is not a compiler: it shells out to `go` for the
+      # `before:` hook and for every one of the four cross-compiles.
+      # Nothing else in this repo puts a Go toolchain on the runner --
+      # check.yml runs script/cibuild, which does all of its work inside
+      # the digest-pinned Dockerfile images -- so without this step the
+      # release either fails at the before-hook or, worse, ships binaries
+      # built by whatever unpinned Go the runner happens to carry.
+      # REPO_POLICIES.md requires every external reference to be pinned,
+      # and script/release already refuses a goreleaser that is not the
+      # pinned build; the compiler that actually produces the artifacts
+      # is the last thing that should be exempt from that.
+      #
+      # go-version-file rather than a literal: go.mod's `go 1.26.1` is
+      # the single source of truth for the toolchain, the same way the
+      # Dockerfile FROM line is the single source of truth for the
+      # linter version that script/lint enforces. It is a three-component
+      # version, so setup-go resolves it exactly -- no silent drift onto
+      # a newer patch release.
+      #
+      # actions/setup-go v5.6.0, 2025-12-15. Pinned by commit sha, like
+      # the checkout above. v5.x is a node20 action, matching the node20
+      # actions/checkout v4 already in use here; the v6/v7 line requires
+      # a node24 runner, which this Gitea runner has never been asked
+      # for and cannot be assumed to provide.
+      - name: Install Go
+        uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff
+        with:
+          go-version-file: go.mod
+          # setup-go's module cache needs a runner-side cache backend.
+          # A release is cut rarely and a cold module download costs
+          # seconds; a release failing because a cache service is absent
+          # costs a re-tag. Off, deliberately.
+          cache: false
+      - name: Install goreleaser
+        run: script/install-goreleaser
+      - name: Release
+        run: script/release
+        env:
+          # RELEASE_TOKEN is a repository Actions secret: a Gitea access
+          # token with write access to this repository's releases (scope
+          # write:repository), owned by an account that can publish here.
+          # It is deliberately not the runner's automatic token, which is
+          # not guaranteed to carry that scope.
+          GITEA_TOKEN: ${{ secrets.RELEASE_TOKEN }}