chore: bump opencode 1.17.15 -> 1.17.20 + pre-flight version-check guidance

- Dockerfile.variant: OPENCODE_VERSION 1.17.15 -> 1.17.20 (latest stable on npm,
  verified via npm view). Variant-only layer; base image untouched.
- AGENTS.md: Pre-flight check now requires checking whether opencode is behind
  and ASKING THE USER before bumping ahead of a release CI build; links the
  bg-subagents removal-trigger re-check.
- CHANGELOG Unreleased: record the bump; 1.17.16-1.17.20 are TUI/cosmetic +
  internal fixes; bg-subagents removal-trigger re-checked, not fired.

CHANGELOG.md

@@ -20,6 +20,13 @@ Tags follow **independent semver** (since `v2.0.0`) — they version *this image
   `OPENCODE_MODEL` example in `.env.example`. Takes effect for new containers
   when no explicit `OPENCODE_MODEL` override is set. The `openai` default
   (`gpt-5.4`) is unchanged.
+- **opencode bumped `1.17.15` → `1.17.20`** (`Dockerfile.variant`
+  `OPENCODE_VERSION`; latest stable on npm, verified via `npm view
+  opencode-ai version`). 1.17.16–1.17.20 are TUI/cosmetic and internal fixes
+  (command-palette flash, sub-agent task-row styling, an OpenAI Responses
+  workaround removal) — no config-affecting or breaking changes. The
+  `OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS` removal-trigger was re-checked
+  against these release notes and has **not** fired; the flag stays baked ON.
 
 ## v2.7.0 — 2026-07-13