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)

CHANGELOG.md

@@ -6,6 +6,19 @@ Tags follow `v{opencode_version}[letter]` — bare tag for the first build on a
 
 ---
 
+## v1.14.31d — 2026-05-01
+
+**CI: collapse per-arch matrix back into single multi-arch push jobs.**
+
+- **Fix:** `v1.14.31c`'s per-arch matrix build jobs failed on `Upload digest` with `GHESNotSupportedError: @actions/artifact v2.0.0+, upload-artifact@v4+ and download-artifact@v4+ are not currently supported on GHES`. Gitea Actions only implements the v3-compatible artifact API; `@v4` uses a GitHub-Enterprise-specific backend. Separately, `build-omos linux/arm64` hung silently for 12 minutes in "Set-up job" and then failed with no log output — likely catthehacker image-pull contention between concurrent matrix children on the same runner host.
+  - Rather than downgrade to `actions/{upload,download}-artifact@v3`, collapsed the per-arch matrix entirely. `docker/build-push-action@v7` with `platforms: linux/amd64,linux/arm64` publishes a proper multi-arch manifest in a single job, so the whole artifact-passing and `imagetools create` merge dance existed only to support a matrix split we no longer need.
+  - The original matrix split was designed around `load: true` disk exhaustion (v1.14.30b). With `push-by-digest`/`push: true` streaming straight to the registry — no local unpack — the peak disk story is fundamentally different. Validated in v1.14.31b that the reclaim step gives sufficient headroom for a single-job amd64 build; oracle-reviewed call that this should extend to the combined amd64+arm64 push case.
+  - Workflow goes from 7 jobs to 5 (smoke-base, smoke-omos, build-base, build-omos, update-description). 263 → ~110 lines of YAML in `docker-publish.yml`.
+- **Add:** `timeout-minutes: 90` on both build jobs so a hung arm64 build produces an explicit failure with logs rather than runner-default silent truncation.
+- **Add:** `BUILDKIT_PROGRESS=plain` at workflow level so arm64-under-QEMU build output is line-by-line (the default collapsed progress UI was obscuring earlier stalls).
+- **Add:** `AGENTS.md §CI quirks` documents the Gitea-specific traps encountered this week: `upload-artifact@v3`-only on Gitea, `/bin/sh` is dash, `build-push-action@v7` does multi-arch natively with comma-separated platforms, reclaim step is mandatory on `load: true` jobs.
+- No image changes. Rebuild of v1.14.31 content only.
+
 ## v1.14.31c — 2026-05-01
 
 **CI: fix bash-specific parameter expansion and bump omos size threshold.**