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.

IDEAS.md

@@ -0,0 +1,60 @@
+# Ideas & backlog
+
+A living list of potential improvements for opencode-devbox that are **not yet
+scheduled**. This is intentionally lightweight — a place to park ideas so they
+aren't lost between sessions. When an item ships, describe it in
+[`CHANGELOG.md`](CHANGELOG.md) and remove it from here.
+
+Rough effort tags: 🟢 small · 🟡 medium · 🔴 large. Status: `idea` (unvetted) ·
+`planned` (agreed, not started).
+
+---
+
+## Supply-chain hardening
+
+- 🟡 `planned` — **Pin CI actions to commit SHAs.** The workflows use floating
+  major tags (`actions/checkout@v4`, `docker/build-push-action@v7`,
+  `docker/setup-buildx-action@v4`, `docker/login-action@v3`,
+  `docker/setup-qemu-action@v3`). This is inconsistent with the project's own
+  philosophy of SHA-pinning *content* refs to defeat floating refs. Pin each
+  action to a SHA with a trailing `# vX.Y.Z` comment. Pairs naturally with the
+  renovate item below to keep the pins fresh.
+
+- 🟡 `planned` — **Vulnerability scanning in CI.** No CVE scan runs on the
+  published images today. Add a `trivy image` (or grype) job to
+  `docker-publish-split.yml` after `smoke`. Start non-blocking (report only),
+  then tighten to fail on `HIGH`/`CRITICAL` with an available fix.
+
+- 🟢🟡 `planned` — **Standardize build provenance → buildx SBOM + attestations.**
+  The image already carries hand-rolled provenance (OCI labels +
+  `build-manifest`). `docker/build-push-action` can emit a standard SBOM and
+  SLSA provenance attestation nearly for free (`provenance: mode=max`,
+  `sbom: true`). Makes provenance machine-consumable and pairs well with the
+  trivy item (scan the SBOM).
+
+## Dockerfile hardening
+
+- 🟡 `idea` — **Address hadolint DL4006 properly.** Currently ignored in
+  `.hadolint.yaml`. The clean fix is `SHELL ["/bin/bash", "-o", "pipefail",
+  "-c"]` so piped `RUN`s fail on the first non-zero stage. This changes the
+  default `RUN` shell from `sh` to `bash` for all subsequent layers, so it is
+  base-affecting and needs a careful pass over existing `RUN`s before removing
+  the ignore.
+
+## Developer experience
+
+- 🟢 `idea` — **`Makefile`/`justfile` for local iteration.** Reproducing a CI
+  build locally means hand-assembling many `--build-arg`s. Thin targets
+  (`make build-base`, `make build-variant`, `make smoke`, `make lint`) would
+  make local testing painless and document the canonical invocations.
+
+- 🟡 `idea` — **Dependency-update automation (renovate).** With CI actions
+  SHA-pinned (above), a `renovate.json` keeps those pins — plus the pinned tool
+  versions (`ACTIONLINT_VERSION`, `HADOLINT_VERSION`, gosu, etc.) — current via
+  automated PRs. Requires a renovate runner against the Gitea instance.
+
+## Housekeeping
+
+- 🟢 `idea` — **Registry retention for `base-<hash>` tags.** The base-hash
+  caching scheme accumulates `base-<hash>` tags over time. Confirm whether the
+  registry prunes old ones, and add a retention/cleanup step if not.