feat: bake OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=true (base ENV)

opencode gates native background subagents behind this experimental flag;
oh-my-opencode-slim V2+ makes background orchestration its default workflow,
so the omos variant was effectively degraded without it. Set as a base-level
ENV (both variants, harmless for plain opencode) and overridable at runtime.

Documented in lockstep: README env table, .env.example, CHANGELOG Unreleased,
and AGENTS.md (with a removal trigger for when opencode promotes it out of
experimental). smoke-test asserts the var is baked into the image env.

Also folds the prior CLI-tools additions into the CHANGELOG Unreleased block.

scripts/smoke-test.sh

@@ -143,6 +143,11 @@ run "graphviz (dot)"    "dot -V"
 run "tldr (tealdeer)"   "tldr --version"
 run "dot-watch"         "test -x /usr/local/bin/dot-watch && bash -n /usr/local/bin/dot-watch && echo ok"
 
+# Background subagents: opencode gates them behind this experimental env var,
+# and OMOS V2+ default orchestration depends on it. Baked ON as an ENV in
+# Dockerfile.base — assert it's present in the image environment (both variants).
+run_expect "bg-subagents env baked" "printenv OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS" "true"
+
 # SSH ControlMaster baked defaults: the config file must exist (image-level)
 # and ssh -G must report ControlPath rooted at /tmp/sshcm/ for an arbitrary
 # host. Catches both regressions: someone removing the conf file, OR something