docs+chore: refresh bg-subagents markers to 1.17.20/omos 2.2.0, link THIRD_PARTY, guard build-args table

- Dockerfile.base + AGENTS.md: bg-subagents removal-trigger last-checked markers
  1.17.15/omos 2.1.0 -> 1.17.20/omos 2.2.0 (re-verified; trigger still not fired).
  The Dockerfile.base comment edit changes the base-content hash, so base-<hash>
  advances and the base image rebuilds on the next release.
- README: refresh stale OPENCODE_VERSION build-arg examples (1.5.0, 1.17.8 -> 1.17.20);
  link THIRD_PARTY.md from the License section.
- AGENTS.md: adding a new floated *_VERSION ARG must also update the README Build Args
  table (closes the coupling gap that hid MICRO/TEALDEER/TYPST for 3 releases).
- CHANGELOG Unreleased: record both under Changed + Documentation.

CHANGELOG.md

@@ -27,10 +27,12 @@ Tags follow **independent semver** (since `v2.0.0`) — they version *this image
   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.
+- **Refreshed the bg-subagents removal-trigger "last-checked" markers** (`Dockerfile.base`, `AGENTS.md`) from `opencode 1.17.15 / omos 2.1.0` to `opencode 1.17.20 / omos 2.2.0`. Comment-only and the trigger has still **not** fired — but the `Dockerfile.base` edit changes the base-content hash, so `base-<hash>` advances and **the base image rebuilds** on the next release (not just the variant).
 
 ### Documentation
 
 - **README backfill for v2.4.0–v2.7.0 features that shipped without user-facing docs:** PDF export via `pandoc --pdf-engine=typst` (new *Document conversion* section), terminal `TERM`/terminfo support (`ncurses-term` + `kitty-terminfo` + `xterm-ghostty`), Neovim 24-bit colour default (`termguicolors` via system `sysinit.vim`), the first-shell host SSH reachability check, and the baked global gitignore. Also adds the three floated build args missing from the Build Args table (`MICRO_VERSION`, `TEALDEER_VERSION`, `TYPST_VERSION`). Docs-only — no image change.
+- **README + AGENTS.md polish:** refreshed stale `OPENCODE_VERSION=` build-arg examples (`1.5.0`, `1.17.8` → the current `1.17.20`), linked the new `THIRD_PARTY.md` from the *License* section, and added an AGENTS.md reminder to add any new floated `*_VERSION` ARG to the README *Build Args* table (the coupling gap that hid `MICRO`/`TEALDEER`/`TYPST` for three releases).
 
 ## v2.7.0 — 2026-07-13