Bump opencode 1.15.4 -> 1.15.6 + cut v1.15.6

Two upstream patch releases since v1.15.4b. Plus this release picks up
two workflow improvements that landed on main between v1.15.4b and
now (b6e4d89 pycache/DS_Store filter in base-decide, 90e5a1f doc-drift
sweep clause in AGENTS.md). No image-content changes beyond the
version bump; cache hit expected on base-35ee5fe7861a since neither
Dockerfile.base nor rootfs/ have changed.

The with-pi and omos-with-pi variants will also implicitly bump pi
0.75.3 -> 0.75.4 because PI_VERSION=latest resolves at build time.
The omos-with-pi smoke threshold (3700 MB after the v1.15.4b bump)
should accommodate two opencode patch versions plus one pi patch
without recurrence of the trip; a future bump-bump pattern would
push it again.

First release on the new CI path that exercises:
- pinned crane install (T14, v1.15.3) - only fires on real base rebuild,
  cache-hit on base means it stays unexercised this run too
- skip promote-base-latest on cache-hit (T15, v1.15.4) - active
- update-description always-and-success-of-base wrap (v1.15.4b) -
  active, will run since base variant publishes

Dockerfile.variant

@@ -32,7 +32,7 @@ ARG USER_NAME=developer
 
 # ── Install opencode via npm ─────────────────────────────────────────
 ARG INSTALL_OPENCODE=true
-ARG OPENCODE_VERSION=1.15.4
+ARG OPENCODE_VERSION=1.15.6
 RUN if [ "${INSTALL_OPENCODE}" = "true" ]; then \
       NPM_CONFIG_PREFIX=/usr npm install -g opencode-ai@${OPENCODE_VERSION} && \
       opencode --version ; \