Collapse per-arch matrix back into single multi-arch push jobs

v1.14.31c's matrix jobs failed on Upload digest with GHESNotSupportedError
— Gitea Actions doesn't support actions/upload-artifact@v4+.
Separately, build-omos arm64 hung silently for 12 min in Set-up job,
likely catthehacker pull contention between concurrent matrix children.

Rather than downgrade artifacts to @v3, collapse the matrix entirely.
docker/build-push-action@v7 with platforms: linux/amd64,linux/arm64
publishes a proper multi-arch manifest in one job, so the
artifact-passing and imagetools create merge dance only existed to
support a matrix split we no longer need.

The matrix was designed around load: true disk exhaustion (v1.14.30b),
but push-by-digest streams straight to the registry with fundamentally
different disk profile. Reclaim step gives enough headroom for the
combined amd64+arm64 push case.

Workflow: 7 jobs → 5. docker-publish.yml: 263 → ~110 lines of YAML.

Also:
- timeout-minutes: 90 on build jobs so hung builds fail explicitly
- BUILDKIT_PROGRESS=plain at workflow level for line-by-line arm64 logs
- AGENTS.md §CI quirks documents the Gitea-specific traps
  (upload-artifact@v3-only, dash-not-bash, build-push-action@v7
  multi-arch convention, reclaim requirement)

AGENTS.md

@@ -47,6 +47,11 @@ When bumping the opencode version, also bump `OPENCODE_VERSION` in `Dockerfile`
 - `update-description` job runs only when both builds succeed (`needs: [build-base, build-omos]`).
 - Tags must be pushed to trigger the publish workflow. The validate workflow runs on push to main and PRs.
 - Smoke tests run on amd64 only (single-arch load into the local daemon). The multi-arch push happens after smoke passes.
+- **Gitea Actions runner has ~40 GB disk, often 70%+ used at job start.** All four `load: true` jobs (`validate-base`, `validate-omos`, `smoke-base`, `smoke-omos`) include a `Reclaim runner disk` step that strips catthehacker-resident toolchains and prunes stale docker state before `setup-buildx-action`. Build jobs use a lighter version (push-by-digest doesn't need `docker system prune`). Don't remove these steps without testing on a fresh runner.
+- **`docker/build-push-action@v7` with `platforms: linux/amd64,linux/arm64` handles multi-arch push natively in a single job** — produces a proper manifest list, no matrix or merge step needed. An earlier revision split into per-arch matrix jobs with digest artifacts, but that pattern requires `actions/{upload,download}-artifact@v4+` which Gitea Actions doesn't support (see below).
+- **`actions/upload-artifact` and `actions/download-artifact` must stay at @v3 on Gitea.** v4+ uses a GitHub-Enterprise-specific Artifact API; runs fail with `GHESNotSupportedError`. If you need artifacts for a new reason (build logs, SBOMs, etc.), pin @v3 explicitly.
+- **Step scripts run under `/bin/sh` (dash), not bash.** Avoid bash-isms like `${VAR//a/b}` parameter-pattern substitution; use POSIX alternatives (`tr`, `sed`) or declare `shell: bash` on the step.
+- **`BUILDKIT_PROGRESS=plain`** is set at workflow level on `docker-publish.yml` so arm64-under-QEMU builds log each layer line-by-line. The default collapsed progress UI hides which step is stalled, which made diagnosing earlier hangs expensive.
 
 ## Testing changes