Add Gitea Actions image-build workflow

Distilled from the house pattern across sibling projects: tag push +
workflow_dispatch triggers, REGISTRY_TOKEN login, raw docker build/push
to gitea.home.hrajfrisbee.cz. Adds a lightweight test gate, an immutable
sha-<12> tag, :latest only on real tag pushes, and a concurrency group.

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

docs/plans-executions/2026-08-11-1935-gitea-build-workflow.md

@@ -0,0 +1,42 @@
+# Execution: Distilled Gitea Actions image-build workflow
+
+Plan: `docs/plans/2026-08-11-1935-gitea-build-workflow.md`
+
+- [x] Step 1 — Create `.gitea/workflows/build.yaml`
+- [x] Step 2 — Replace CLAUDE.md CI TODO with a CI/CD subsection
+- [ ] Step 3 — Push branch + open MR
+- [ ] Step 4 — CHANGELOG entry (after the first successful run is confirmed)
+
+## Steps 1–2 — workflow + CLAUDE.md
+
+The workflow distills the house pattern from 9 sibling projects (survey in the plan)
+plus improvements none of them combine: an immutable `sha-<12>` tag, a lightweight
+test gate, `:latest` moving only on real tag pushes, and a `concurrency` group.
+
+Work happened in a git worktree off `origin/main` so the main checkout (which had
+unrelated uncommitted changes) stayed untouched:
+
+```bash
+git worktree add -b feat/gitea-build-workflow \
+  "$SCRATCH/wt-build" origin/main
+```
+
+Deviation from the plan's assumptions: while planning, `feat/proxy-operator` was
+still unmerged and the plan noted `main` lacked `internal/`/`test/`. By execution
+time `origin/main` had moved (`076bc66..f7000f7` — the proxy-operator MR merged),
+so the branch and its CI gate cover the full operator code.
+
+Verified the workflow parses and the check-gate commands pass on this exact tree
+(ruby stands in for a YAML linter because the system python3 has no `yaml` module):
+
+```bash
+ruby -ryaml -e "YAML.load_file('.gitea/workflows/build.yaml'); puts 'YAML OK'"
+go vet ./... && go build ./... && go test -short ./...   # all packages ok
+```
+
+Worth noting: the workflow itself cannot run end-to-end until (a) the MR merges
+(it only triggers on tags / manual dispatch, not branch pushes) and (b) the
+`REGISTRY_TOKEN` secret is created in this repo's Gitea settings (PAT with
+`write:package`). First real verification = manual dispatch with tag
+`manual-test`, expecting `manual-test` + `sha-…` in Packages and `:latest`
+untouched.