p5-03: docker-only release path (drop goreleaser)

Single public deliverable per tag: a multi-arch server image, with
cross-compiled agent binaries + install scripts + the systemd unit
baked under /opt/restic-manager/dist/. The /agent/binary and
/install/* handlers fall back from <DataDir>/... to that read-only
path so a fresh container Just Works without first-run staging;
operators can still drop a custom build into <DataDir>/ to override
per-host.

Architecture rationale: agent distribution already routes through
the running server, so the release surface mirrors that — there's
no second source of truth to keep in sync.

Workflow .gitea/workflows/release.yml triggers on v*.*.* tag-push
(fan-out :vX.Y.Z / :X.Y / :X, plus :latest once MAJOR>=1) and
workflow_dispatch (snapshot tag only). Pushes to the Gitea
container registry on this instance.

Both binaries grow main.commit + main.date ldflag targets. Makefile
and Dockerfile fill them; release workflow forwards from gitea.sha
plus a UTC timestamp.

Spec : docs/superpowers/specs/2026-05-05-p5-03-docker-only-release.md
Plan : docs/superpowers/plans/2026-05-05-p5-03-docker-only-release.md

tasks.md

@@ -328,7 +328,7 @@ Sizes: **S** = under a day, **M** = 1–3 days, **L** = 3–7 days.
 
 - [ ] **P5-01** (M) Documentation site (mdBook or similar) with install, concepts, security model, screenshots
 - [ ] **P5-02** (S) `CONTRIBUTING.md`, `CODE_OF_CONDUCT.md`, issue + PR templates
-- [ ] **P5-03** (S) Release automation: `goreleaser` for binaries + Docker image to GHCR
+- [x] **P5-03** (S) Release automation — **pivoted away from goreleaser/binary archives** on 2026-05-05 (spec: `docs/superpowers/specs/2026-05-05-p5-03-docker-only-release.md`). Single deliverable per tag: a multi-arch (linux amd64+arm64) server image, with cross-compiled agent binaries (linux amd64+arm64, windows amd64) + `install.sh` + `install.ps1` + the systemd unit baked under `/opt/restic-manager/dist/`. The `/agent/binary` and `/install/*` handlers fall back from `<DataDir>/...` to `<BundledAssetsDir>/...` so a fresh container Just Works. Workflow `.gitea/workflows/release.yml` triggers on `v*.*.*` tag-push (real release: fan-out `:vX.Y.Z`, `:X.Y`, `:X`, plus `:latest` once `MAJOR>=1`) and `workflow_dispatch` (snapshot: `:snapshot-<shortsha>` only). Pushed to the Gitea container registry on this instance — no external creds, no GHCR mirror. Cosign / SBOM / minisign / GHCR mirror deferred to Phase 6. Source builds via `make build` remain a first-class path.
 - [ ] **P5-04** (S) Demo screenshots / short Loom walkthrough in README
 - [ ] **P5-05** (S) `SECURITY.md` with disclosure process
 - [ ] **P5-06** (M) End-to-end test suite in CI (Playwright vs. compose stack with sibling Linux agent)