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.

MEMORY.md

@@ -142,9 +142,13 @@ Session memory for durable decisions, state, and gotchas. Keep concise, remove o
   job ended "could not create or find release". The repo also has **no** `GITEA_TOKEN`
   secret (`/actions/secrets` is empty) and the runner has no git credentials. Fixes: the
   script sets `AUTH=(-u "$CRED")`; the workflow falls back to `github.token`
-  (`secrets.GITEA_TOKEN || github.token`). **v0.2.9 is the only release after v0.2.1**
-  (built and published locally, all five artifacts); v0.2.4–v0.2.8 tags have no release
-  artifacts. Verify the CI fallback on the next tag push. PLAN.md + ADRs 0007–0010 still describe the intended UX.
+  (`secrets.GITEA_TOKEN || github.token`) — but `github.token` is **not** provided by this
+  Gitea/act runner, so the env dump showed no token and the v0.2.10 tag build failed again.
+  Final fix: `publish_release.sh` falls back to the **local git extraheader** that
+  `actions/checkout` writes (`git config --local --get-regexp '^http\..*\.extraheader$'`),
+  which carries the runner's basic-auth credentials for the same server. Verified locally by
+  adding that extraheader and publishing v0.2.10 (all five artifacts). **v0.2.9 and v0.2.10
+  are published**; v0.2.4–v0.2.8 tags have no artifacts. Verify the CI path on the next tag. PLAN.md + ADRs 0007–0010 still describe the intended UX.
 - **Release v0.2.1 is LIVE** (2026-10-31): CI run 1110 built the NSIS installer + AppImage
   and published — installers/AppImage are on the releases page. Releasing = bump version → tag → push.
 - NSIS gotchas (validated in a local ubuntu:24.04 docker repro): makensis uses `-D` defines