Migrate CI to shared org-level Tauri CI base image

[ScottMorris]

Context

test-ci.yml currently uses a per-repo CI base image:

container:
  image: ghcr.io/scottmorris/liminal-notes-ci-base-test:latest

A shared org-level image is being created at ghcr.io/liminal-hq/tauri-ci-base (tracked in liminal-hq/tauri-plugins-workspace).

Proposal

1. Update test-ci.yml container image to ghcr.io/liminal-hq/tauri-ci-base:latest
2. Update .devcontainer/Dockerfile to use the shared image as base
3. Retire build-ci-images.yml once migration is verified
4. Note: if the shared image is private, a PAT with read:packages scope is needed for cross-org access. If public, no changes needed.

Acceptance Criteria

• test-ci.yml references the shared org image
• .devcontainer uses shared image as base
• CI passes with the shared image
• build-ci-images.yml removed or repurposed