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.

TODO.md

@@ -31,6 +31,8 @@ Verified via API+host credentials (2026-10-31):
       so every tag build since v0.2.2 failed to publish.
 - [x] Add a `github.token` fallback in `.gitea/workflows/build.yml` (the repo has no
       `GITEA_TOKEN` secret).
+- [x] Fall back to the checkout's git extraheader when no token/credentials exist (the
+      Gitea runner does not expose `github.token`).
 - [ ] Verify the fallback on the next tag push produces a release automatically.
 - [ ] Optionally backfill releases for v0.2.4–v0.2.8 (their CI artifacts were not
       retained; would need local rebuilds) or leave them superseded by the latest.