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.

Dockerfile.base

@@ -403,6 +403,17 @@ ENV LANGUAGE=en_US:en
 ENV LC_ALL=en_US.UTF-8
 ENV EDITOR=nvim
 ENV PATH="/home/developer/.local/bin:/home/developer/.cargo/bin:${PATH}"
+# Enable opencode's native background subagents. opencode gates this behind an
+# experimental flag (packages/opencode/src/tool/task.ts errors with
+# "Background subagents require OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=true"
+# when unset). oh-my-opencode-slim V2+ makes background orchestration its DEFAULT
+# workflow, so the omos variant is effectively broken without this. Baked here as
+# a base ENV (applies to both variants; harmless for plain opencode — it only
+# *enables* a capability). Overridable at runtime: -e OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=false.
+# REMOVAL TRIGGER: when opencode promotes background subagents out of experimental
+# (flag becomes default / renamed), drop this ENV. No upstream roadmap date as of
+# opencode 1.17.15 / omos 2.1.0 (2026-07).
+ENV OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=true
 
 # ── Node.js (required for opencode/pi/omos at variant build + MCP servers) ──
 ARG NODE_VERSION=22