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.

.env.example

@@ -117,3 +117,9 @@ SSH_KEY_PATH=~/.ssh
 #                              # on image pull. Independent of ENABLE_OMOS.
 #                              # See docs/omos-skills.md
 # OMOS_RESET=false             # Force regenerate oh-my-opencode-slim config on next start (does not affect skills)
+
+# ── Background subagents (opencode experimental flag) ────────────────
+# Baked ON in the image (ENV in Dockerfile.base) because OMOS V2+ default
+# orchestration depends on opencode's native background subagents. Uncomment
+# to opt out (e.g. to force blocking/foreground orchestration).
+# OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=false