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.

CHANGELOG.md

@@ -6,6 +6,35 @@ Tags follow **independent semver** (since `v2.0.0`) — they version *this image
 
 ---
 
+## Unreleased
+
+CLI-toolset parity with the sibling `pi-devbox` image, plus background-subagent
+enablement. Touches `Dockerfile.base` (apt list, new binary, new ENV) so the
+base image rebuilds (`base-<hash>` advances).
+
+### Added
+
+- **CLI tooling parity with pi-devbox.** Added `yq` (YAML query/transform,
+  pairs with `jq`), `pandoc` (Markdown↔HTML/etc. document converter), and
+  `graphviz` (`dot` diagram rendering) via apt; `tealdeer` (the `tldr` command —
+  quick command examples) as a ~5 MB static musl binary via the floated
+  `TEALDEER_VERSION` arg; and the `dot-watch` wrapper (`rootfs/usr/local/bin/dot-watch`,
+  auto-renders a `.dot` file to PNG on save, graphviz-only). Deliberately did
+  **not** port `socat`, `imagemagick`, or the pi-studio-specific `studio-expose`
+  bridge. smoke-test asserts presence of all five.
+- **Background subagents enabled by default (`OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=true`).**
+  Baked as a base-level `ENV` in `Dockerfile.base`. opencode gates native
+  background subagents behind this experimental flag; OMOS V2+ makes background
+  orchestration its default workflow, so the omos variant was effectively
+  degraded without it. Overridable at runtime (`-e OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=false`).
+  Documented in README env table and `.env.example`; smoke-test asserts the var
+  is present in the image environment. Marked experimental upstream — carries a
+  removal-trigger note in `Dockerfile.base` + AGENTS.md for when opencode
+  promotes it out of experimental (no upstream roadmap date as of opencode
+  1.17.15 / omos 2.1.0).
+
+---
+
 ## v2.5.0 — 2026-07-02
 
 Minor release. Headline: **optional shared/external MemPalace** —