Cut v1.15.12 — opencode 1.15.11→1.15.12 upstream, pi 0.76.0→0.77.0

opencode-ai actually released 1.15.12 upstream (2026-05-28); this is
the genuine first container build on it, plus the pi 0.77.0 bump
(Claude Opus 4.8, --exclude-tools, headless Codex subscription login,
streaming-aware extension input, plus a long fix list).

Re-uses the v1.15.12 git tag, force-overwriting the historical
artifact tag at be2a168 from the 2026-05-28 versioning slip (caught
same day and re-cut as v1.15.11c; corresponding Hub images already
manually deleted). Commit be2a168 and the v1.15.11c CHANGELOG block
referencing the slip remain in history.

No base-image change — unchanged Dockerfile.base/rootfs/entrypoint
will hit base-decide cache-hit short-circuit; only the four variant
builds + manifest tagging will run.

See CHANGELOG v1.15.12 for the full upstream notes.

Dockerfile.variant

@@ -36,7 +36,7 @@ ARG USER_NAME=developer
 # 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=1.15.11
+ARG OPENCODE_VERSION=1.15.12
 RUN if [ "${INSTALL_OPENCODE}" = "true" ]; then \
       NPM_CONFIG_PREFIX=/usr npm install -g opencode-ai@${OPENCODE_VERSION} && \
       opencode --version ; \