feat: pi-devbox parity — typst PDF, terminal terminfo, nvim colour, host-ssh check, hygiene

Ports the base additions from pi-devbox v1.4.0 + v1.5.0 that opencode-devbox
lacked (opencode-devbox already tracks pi-devbox for CLI-toolset parity, v2.6.0):

- typst PDF engine for pandoc (v1.4.0) + the pandoc typst-template default-font
  patch (v1.5.0) so 'pandoc --pdf-engine=typst' works without -V mainfont.
  pandoc shipped since v2.6.0 as a front-end only (no PDF back-end). +xz-utils.
  Tracks latest; --build-arg TYPST_VERSION escape hatch.
- Terminal support (v1.5.0): ncurses-term + kitty-terminfo + a compiled
  xterm-ghostty alias (tic -x, use=ghostty), so wezterm/alacritty/foot/ghostty/
  kitty resolve TERM over SSH instead of degrading to a dumb fallback.
- Readable Neovim colours (v1.5.0): system-wide /etc/xdg/nvim/sysinit.vim with
  termguicolors.
- Host SSH reachability check at shell startup (v1.4.0): one-time probe in
  .bash_aliases warning (with fix steps + inline pubkey) when the Mac host is
  unreachable. The rest of the LAN stack was already present.
- .claude/settings.local.json added to the gitignore_global seed (v1.5.0).
- Repo hygiene (v1.5.0): LICENSE (MIT), THIRD_PARTY.md, hadolint CI job (pinned
  v2.14.0) + .hadolint.yaml, IDEAS.md backlog.

Base-affecting (Dockerfile.base + rootfs) → base-<hash> advances, base rebuilds.
smoke-test gains typst/PDF, terminfo, and nvim-tgc assertions. Validated:
hadolint clean on both Dockerfiles, bash -n OK, base-hash guard OK, workflow
guard OK. CHANGELOG v2.7.0.

THIRD_PARTY.md

@@ -0,0 +1,48 @@
+# Third-party notices
+
+opencode-devbox is distributed under the MIT License (see [`LICENSE`](LICENSE)),
+which covers **this repository's own contents** — the Dockerfiles, entrypoint
+scripts, `rootfs/` seeds, CI workflows, and docs.
+
+The **published container images** (`joakimp/opencode-devbox:*`) additionally
+*bundle* third-party software, each of which remains under its own license. This
+file is a good-faith summary; the authoritative sources are the upstream
+projects and, for OS packages, the per-package copyright files inside the image
+at `/usr/share/doc/<package>/copyright`.
+
+## opencode and agent components (installed in the variant layer)
+
+| Component | Upstream | License |
+| --- | --- | --- |
+| opencode | github.com/sst/opencode | MIT |
+| oh-my-opencode-slim (OMOS) *(`-omos` variant only)* | npm | MIT |
+| mempalace / mempalace-toolkit | authored by the maintainer (Joakim Persson) | MIT |
+| gitea-mcp | gitea.com/gitea/gitea-mcp | MIT |
+
+## Tooling baked into the base image
+
+| Component | Upstream | License (best effort) |
+| --- | --- | --- |
+| gosu | github.com/tianon/gosu | Apache-2.0 |
+| Node.js | nodejs.org | MIT (bundles components under their own licenses) |
+| uv | github.com/astral-sh/uv | Apache-2.0 OR MIT |
+| Neovim | neovim.io | Apache-2.0 + Vim license |
+| micro | github.com/zyedidia/micro | MIT |
+| Pandoc | pandoc.org | GPL-2.0-or-later |
+| Typst | github.com/typst/typst | Apache-2.0 |
+| Graphviz | graphviz.org | CPL-1.0 |
+| ripgrep / fd / bat / eza / zoxide / tealdeer / yq (mikefarah) | respective repos | MIT / Apache-2.0 / Unlicense (varies) |
+| bun *(`-omos` variant only)* | github.com/oven-sh/bun | MIT |
+
+## Base OS
+
+The image is built `FROM` a Debian base and installs packages via `apt`. Debian
+and its packages are distributed under their respective licenses (GPL, LGPL,
+MIT, BSD, and others). See each package's copyright file in the image under
+`/usr/share/doc/<package>/copyright`.
+
+---
+
+*Licenses marked "best effort" are widely known but were not each verified at
+the exact bundled version; consult the upstream project for authoritative
+terms. Corrections welcome.*