ci: workflow-level 3-attempt retry around buildx build --push

Belt-and-braces against transient registry-1.docker.io blips (rate
limits, brief 5xx, CDN flap). Replaces all five push docker/build-push-
action@v7 invocations (1 base + 4 variants) with shell: bash steps that
run docker buildx build --push in a for-loop with backoff (15s, 30s).
Smoke build steps (load: true, no push) are untouched.

Does NOT mask deterministic failures: a true regression (e.g. the
cache-export 400 we hit 2026-05-23..28) fails all 3 attempts
identically and the job still fails by design. Orthogonal layer to
both cache-export disablement and the ci-release-watcher skill's
transient-rerun heuristic.

- AGENTS.md: new Critical conventions bullet documenting the retry
  pattern, the consistency rule across push steps, and why it's
  duplicated rather than factored (Gitea Actions doesn't support
  reusable composite shell steps cleanly).
- CHANGELOG.md: Unreleased section addendum, no image-side change.

No image-side change.

AGENTS.md

@@ -74,6 +74,7 @@ cd /tmp && npm pack @earendil-works/pi-coding-agent@0.75.5 && tar -xzf earendil-
 - **Resolved versions are logged by the smoke test** — `scripts/smoke-test.sh` prints a "Resolved component versions" table as its first step. CI logs always capture what got baked into a given image even when ARGs default to `latest`.
 - **`PI_VERSION` and `OMOS_VERSION` MUST be passed by CI as concrete versions**, not left at the `latest` default. The npm install steps in `Dockerfile.variant` (`npm install -g @earendil-works/pi-coding-agent` / `oh-my-opencode-slim@${OMOS_VERSION}`) produce identical layer-hashes when the ARG values are byte-identical across builds; combined with the registry buildcache (`base-buildcache`) the layer gets reused even when `latest` would have resolved to a newer upstream. This is the same class of bug that bit pi-devbox v0.74.0 → v0.75.5 (silent same-bytes-across-releases regression discovered 2026-05-23, fixed in pi-devbox v0.75.5b). It is currently *masked* in opencode-devbox by `OPENCODE_VERSION` being a hard-coded ARG that bumps every release — that bump invalidates the parent-chain cache key for the downstream pi/omos layers — but the masking would fail the moment a `vN.N.Nb` opencode-version-unchanged release ships that only bumps pi or omos. Preventative fix: `.gitea/workflows/docker-publish-split.yml` has a `resolve-versions` job that runs `npm view @earendil-works/pi-coding-agent version` and `npm view oh-my-opencode-slim version`, exposing concrete values as outputs that every variant smoke + build job consumes via build-args. Smoke tests assert via `EXPECTED_PI_VERSION` / `EXPECTED_OMOS_VERSION` env vars — would catch the regression on the next release rather than four releases later. **If you change the variant build-args list, the resolve-versions job, or the smoke EXPECTED_*_VERSION wiring, audit all affected jobs in lockstep.**
 - **Registry buildkit cache-export is currently disabled** — do NOT re-add `cache-from`/`cache-to` to the `build-base` step in `.gitea/workflows/docker-publish-split.yml` without first verifying that buildkit's `mode=max` cache-export to `registry-1.docker.io` no longer returns HTTP 400 from the Hub CDN edge. The regression surfaced ~2026-05-23 and broke five consecutive opencode-devbox publish attempts (runs #332/333/334/336 + a rerun); root-caused on 2026-05-28 by a manual host-side publish that reproduced the same 400 only on `--cache-to` while image push worked fine. Failure shape is stable (`Offset:0` in the `_state` token, HTML response body = CDN-tier rejection, not registry backend), repo-specific (we're the only repo writing `:base-buildcache` mode=max), and explains why pinning `setup-buildx-action@v4.0.0` didn't help (action pin doesn't change the bundled buildkit version on the catthehacker runner image). Trade-off: dockerfile.base changes pay a full ~3 min rebuild instead of pulling cached layers; unchanged bases short-circuit at the Hub-probe step in `base-decide` and never re-build anyway. Variants don't use registry cache so they're unaffected. Re-enable condition: upstream moby/buildkit fix lands AND a low-risk test run succeeds without 400s. See CHANGELOG v1.15.12 `Unreleased` block for the full diagnostic chain. Manual escape-hatch publish procedure: `docs/manual-host-publish.md`.
+- **Push steps wrap `docker buildx build --push` in a 3-attempt retry loop** (15s, 30s backoff) for transient `registry-1.docker.io` blips — rate limits, brief 5xx, CDN flap. Implemented as inline `shell: bash` steps with `docker buildx build` raw rather than `docker/build-push-action@v7` so the loop is visible and tweakable. Affects the 1 base + 4 variant push steps in `.gitea/workflows/docker-publish-split.yml`; smoke-test builds (`load: true`, no push) are untouched. **This does NOT mask deterministic failures** — a true regression (like the cache-export 400 of 2026-05-23..28) fails all 3 attempts identically and the job still fails. Orthogonal to the cache-export disablement above: cache-export was about a deterministic protocol mismatch, retry is about absorbing genuine transients. Both are belt-and-braces with the `ci-release-watcher` skill's transient-rerun heuristic. If you change the matrix of push steps, keep the retry wrapper consistent across them — the pattern is duplicated rather than factored out because Gitea Actions doesn't support reusable composite shell steps cleanly.
 - **Shell scripts use `set -euo pipefail`** — both entrypoints are strict. Errors in volume chown or SSH permission operations are intentionally suppressed with `|| true`.
 - **MemPalace install path** — installed via `uv tool install` into `/opt/uv-tools/mempalace/`. Both the `mempalace` CLI and the `mempalace-mcp` MCP server binary are shipped as entry points by the mempalace package itself and placed on PATH by uv as shims whose shebangs point at the venv's Python. No hand-rolled wrapper is needed. Do not use `pip install --break-system-packages` — that was the previous approach and has been removed. Do not use `["python3", "-m", "mempalace.mcp_server"]` in `opencode.jsonc` — system Python can't import from the uv venv.
 - **generate-config.py idempotency** — the script MUST never overwrite an existing `opencode.jsonc` or legacy `opencode.json`. Config persists in the `devbox-opencode-config` named volume; accidentally clobbering that file would destroy hand-edits. The smoke test asserts this.