CI: preventative fix for PI_VERSION/OMOS_VERSION cache-hit silent regression

Mirrors the pi-devbox v0.75.5b fix (2026-05-23) onto the four-variant
pipeline here. The with-pi, omos, and omos-with-pi variants install
upstream npm packages whose *_VERSION build-args defaulted to 'latest'.
When the build-arg string is byte-identical across builds, the layer
hash is identical and the registry buildcache silently reuses the layer
from whatever upstream version was current when the cache was first
populated — same mechanism that shipped pi-devbox v0.74.0..v0.75.5 with
identical image bytes.

Currently masked here because OPENCODE_VERSION is a hard-coded ARG that
bumps every release; parent-chain cache invalidation flushes the
downstream pi/omos layers. Masking would fail on any vN.N.Nb opencode-
version-unchanged release that only bumps pi or omos. Filed last night
as parked followup; fixing preventatively now that #5 (AWS SSO inside
tor-ms22 container) cleared.

CHANGES

.gitea/workflows/docker-publish-split.yml — new resolve-versions job
running 'npm view @earendil-works/pi-coding-agent version' and
'npm view oh-my-opencode-slim version', exposing concrete strings as
job outputs. All six affected jobs (smoke-omos, smoke-with-pi,
smoke-omos-with-pi, build-variant-omos, build-variant-with-pi,
build-variant-omos-with-pi) now consume them as PI_VERSION /
OMOS_VERSION build-args. smoke-base / build-variant-base unaffected.

scripts/smoke-test.sh — new run_expect helper asserting an expected
substring in command output. The pi check uses EXPECTED_PI_VERSION;
the omos check uses EXPECTED_OMOS_VERSION against npm ls -g. Both env
vars are wired from resolve-versions outputs in the smoke jobs. Catches
this regression class on the next release, not four releases later.

Dockerfile.variant — comment blocks above OPENCODE_VERSION (source-
pinned, not subject to the bug), PI_VERSION (CI-resolved), and
OMOS_VERSION (CI-resolved) explaining the cache-hit footgun.

AGENTS.md — new convention bullet under 'Critical conventions' naming
the resolve-versions job + EXPECTED_*_VERSION wiring as the contract
to keep in lockstep when modifying variant build-args.

.gitea/README.md — Step 1 expanded to cover the parallel resolve-
versions job alongside base-decide; pipeline diagram updated.

CHANGELOG.md — Unreleased entry describing the fix, masking mechanism,
and audit footprint.

No image-content change expected on the next release vs what 'latest'
would have resolved to anyway. Purely makes the cache invalidate
correctly going forward.

Dockerfile.variant

@@ -31,6 +31,10 @@ ARG TARGETARCH
 ARG USER_NAME=developer
 
 # ── Install opencode via npm ─────────────────────────────────────────
+# OPENCODE_VERSION is intentionally pinned in this Dockerfile (not
+# 'latest'). It drives the release tag and gets bumped via a source
+# edit, so the cache-hit class of bug that bit pi-devbox v0.74.0..
+# v0.75.5 cannot apply here.
 ARG INSTALL_OPENCODE=true
 ARG OPENCODE_VERSION=1.15.10
 RUN if [ "${INSTALL_OPENCODE}" = "true" ]; then \
@@ -42,6 +46,18 @@ RUN if [ "${INSTALL_OPENCODE}" = "true" ]; then \
 # pi-toolkit and pi-extensions are cloned into /opt/. entrypoint-user.sh
 # runs each repo's install.sh on container start so symlinks land under
 # ~/.pi/agent/ on the named volume.
+# PI_VERSION should be passed explicitly by CI as a concrete version
+# (resolved from `npm view @earendil-works/pi-coding-agent version`,
+# see .gitea/workflows/docker-publish-split.yml § resolve-versions).
+# The default `latest` is for local dev convenience only — it has a
+# known cache-hit footgun when used in registry-cached CI builds: the
+# resulting build-arg string is byte-identical across builds, the
+# layer-hash is identical, and the registry buildcache silently reuses
+# the layer from whatever pi version was current when the cache was
+# first populated. Currently masked here because OPENCODE_VERSION (a
+# parent layer) bumps every release; will manifest the moment a
+# vN.N.Nb opencode-version-unchanged release ships. See pi-devbox
+# v0.75.5b 2026-05-23 for the discovery + canonical fix.
 ARG INSTALL_PI=false
 ARG PI_VERSION=latest
 ARG PI_TOOLKIT_REF=main
@@ -89,6 +105,10 @@ RUN if [ "${INSTALL_GO}" = "true" ]; then \
 
 # ── Optional: oh-my-opencode-slim (multi-agent orchestration) ────────
 # Installs Bun runtime and the oh-my-opencode-slim npm package.
+# OMOS_VERSION shares the same cache-hit footgun as PI_VERSION when
+# left at the `latest` default in registry-cached CI builds. CI
+# resolves it via `npm view oh-my-opencode-slim version` and passes
+# the concrete value as a build-arg. See PI_VERSION block above.
 ARG INSTALL_OMOS=false
 ARG OMOS_VERSION=latest
 RUN if [ "${INSTALL_OMOS}" = "true" ]; then \