feat: add pi-only variant (pi without opencode) as basis for pi-devbox

All opencode-devbox variants set INSTALL_OPENCODE=true, so pointing pi-devbox
at with-pi dragged opencode along and made it ~a re-tag of latest-with-pi.
Add a 5th variant pi-only (INSTALL_OPENCODE=false, INSTALL_PI=true): pi +
companions (toolkit, extensions, fork, recall) + base tooling, no opencode
(~145 MB lighter than with-pi).

- Dockerfile.variant: document pi-only in the variant table.
- CI docker-publish-split.yml: new smoke-pi-only + build-variant-pi-only jobs
  (tags :VERSION-pi-only / :latest-pi-only, multi-arch); wired into
  promote-base-latest and update-description needs.
- validate.yml: new validate-pi-only main-branch gate job.
- smoke-test.sh: accept --variant pi-only; threshold 2750 MB; opencode-absent
  path already handled.
- Docs: HUB_TEMPLATE (regenerated DOCKER_HUB.md), README, AGENTS (variant/tag
  counts 4->5, 8->10 tags), .gitea/README, manual-host-publish.sh (5 variants),
  plan doc implementation note.

This is the single source of truth for joakimp/pi-devbox, which now FROMs
latest-pi-only. Versions unchanged (opencode 1.15.13, pi 0.78.0).

README.md

@@ -463,7 +463,7 @@ All six agents should respond if your provider authentication is working.
 
 ### Setup
 
-Pre-built pi-enabled images are available on Docker Hub as `joakimp/opencode-devbox:latest-with-pi` (base + pi) and `joakimp/opencode-devbox:latest-omos-with-pi` (OMOS + pi). Pulling one of those tags is the fastest path. Alternatively, build from source:
+Pre-built pi-enabled images are available on Docker Hub as `joakimp/opencode-devbox:latest-with-pi` (base + pi) and `joakimp/opencode-devbox:latest-omos-with-pi` (OMOS + pi). Pulling one of those tags is the fastest path. There is also a `latest-pi-only` variant (pi **without** opencode, `INSTALL_OPENCODE=false`) — it's the lean basis for the separate [`joakimp/pi-devbox`](https://gitea.jordbo.se/joakimp/pi-devbox) image. Alternatively, build from source:
 
 ### Build