Publish releases with the runner's checkout credentials

The v0.2.10 tag build still failed to publish: its env dump showed no token at all,
because this Gitea/act runner does not provide `github.token`, and the repo has no
GITEA_TOKEN secret.

publish_release.sh now resolves credentials in order: GITEA_TOKEN,
GITEA_AUTH_HEADER, ~/.git-credentials, then the local git extraheader that
actions/checkout writes for the job. That header carries the runner's basic-auth
credentials for the same server, so tag builds publish with no repo secret.

Verified by adding the extraheader locally and publishing v0.2.10 (all five
artifacts: Linux/Windows/macOS ports, NSIS installer, AppImage), then removing it.

Docs: RELEASING describes the four credential sources and warns that github.token is
not available here; MEMORY records both failed attempts and the final fix; TODO keeps
the "verify on the next tag" follow-up. Version 0.2.11.

docs/RELEASING.md

@@ -37,10 +37,17 @@ The repository ships a Gitea Actions workflow (`.gitea/workflows/build.yml`):
   Linux/Windows/macOS, artifact upload, and — on tags — release publication via
   `tools/publish_release.sh`.
 
-For tag builds the workflow uses `GITEA_TOKEN` (write:repository scope) or, when the
-secret is absent, falls back to the workflow's built-in `github.token`. Either is
-enough for the publish step; with neither, the build succeeds but no release is
-created.
+`tools/publish_release.sh` picks the first available credential, in order:
+
+1. `GITEA_TOKEN` (repo secret, write:repository scope),
+2. `GITEA_AUTH_HEADER` (a ready-made `Authorization: ...` header),
+3. `~/.git-credentials` for the host,
+4. the **local git extraheader** that `actions/checkout` configures on the CI job.
+
+Option 4 is what makes tag builds publish without any repo secret: the runner
+already injects basic-auth credentials for the checkout, and the script reuses that
+header for the API (same server). `github.token` is *not* available in this Gitea
+setup, so do not rely on it.
 
 > **Incident (2026-10-31):** `tools/publish_release.sh` never set `AUTH` in its
 > `~/.git-credentials` branch, so every API call failed silently (`AUTH[@]` was