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.

.hadolint.yaml

@@ -0,0 +1,27 @@
+# hadolint configuration for opencode-devbox.
+#
+# Both Dockerfiles are linted in CI (.gitea/workflows/lint.yml → `hadolint`
+# job). hadolint reads this file automatically, so a local
+# `hadolint Dockerfile.base` reproduces CI exactly.
+#
+# The ignores below are DELIBERATE project choices — they mirror the
+# philosophy of the shellcheck excludes already applied to `run:` steps
+# (SHELLCHECK_OPTS in lint.yml). Anything NOT listed here still fails the
+# build at `warning` and above, so new Dockerfile smells are caught going
+# forward.
+ignored:
+  - DL3008  # "pin apt versions" — intentionally unpinned: the base tracks
+            #   Debian stable and runs `apt-get upgrade`, so pinning point
+            #   versions would rot and fight security updates.
+  - DL3016  # "pin npm versions" — opencode / oh-my-opencode-slim ARE pinned,
+            #   but via build-args (CI-resolved from npm), not the npm CLI.
+  - DL4006  # "set -o pipefail before a pipe" — the piped RUNs are
+            #   download|extract steps with their own retries / `set -e`.
+            #   Switching the global SHELL to bash is a larger, base-affecting
+            #   change — tracked in IDEAS.md.
+  - DL3003  # "use WORKDIR, not cd" — cosmetic in the few `cd` RUNs here.
+  - SC2086  # "double-quote to prevent word-splitting" — the same code is
+            #   excluded for shell `run:` steps in lint.yml; splitting is
+            #   intentional in these contexts.
+
+failure-threshold: warning