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.

README.md

@@ -148,6 +148,7 @@ docker compose exec -u developer devbox aws --version
 | `OMOS_TMUX` | Enable tmux pane integration for OMOS | `false` |
 | `OMOS_SKILLS` | Symlink bundled OMOS skills from the image into `~/.agents/skills/` each start | `true` |
 | `OMOS_RESET` | Force regenerate OMOS config on next start (does not affect skills) | `false` |
+| `OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS` | Enable opencode's native background subagents. Baked on in the image because OMOS V2+ default orchestration depends on it. Set `false` to opt out. opencode marks this **experimental** — see [AGENTS.md](AGENTS.md) removal trigger | `true` |
 | `SKILLSET_CONTAINER_PATH` | Path to skillset repo inside container (for auto-deploy when not at /workspace/skillset) | Auto-detect |
 
 ### Reaching your LAN from the container