docs(readme): backfill user docs for v2.4.0-v2.7.0 features

Features that shipped without README prose are now documented:
- PDF export (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)
- first-shell host SSH reachability check (Shell defaults list)
- baked global gitignore (patterns + core.excludesFile wiring)
- Build Args table: add MICRO_VERSION, TEALDEER_VERSION, TYPST_VERSION

Docs-only; noted under CHANGELOG Unreleased. Hub template and .env.example
were already accurate. Drift found via a v2.4.0+ CHANGELOG-vs-docs audit.

CHANGELOG.md

@@ -28,6 +28,10 @@ Tags follow **independent semver** (since `v2.0.0`) — they version *this image
   `OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS` removal-trigger was re-checked
   against these release notes and has **not** fired; the flag stays baked ON.
 
+### 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.
+
 ## v2.7.0 — 2026-07-13
 
 Minor release. Headline: **further parity with the sibling `pi-devbox` image** —