Removed GHOST_CDN_URL from CI admin build

[rob-ghost]

Context

This is PR 3 of 3 in the deploy-time admin rewrite initiative.

Proposal: Build once, deploy everywhere — deploy-time index.html rewrite

Summary

Ghost(Pro) currently bakes GHOST_CDN_URL=https://assets.ghost.io/admin-forward/ into the admin build at CI time, producing absolute CDN URLs in the artifact. This means Ghost-Moya must string-replace production URLs with staging URLs at deploy time — a fragile process that couples the build to the deployment target.

The new approach builds admin with relative paths (./assets/...) and a ghost-cdn-url meta tag. At deploy time, a simple sed command in Ghost-Moya rewrites index.html per environment — no Node.js script, no string replacement across JS bundles.

What this PR does

• Removes the GHOST_CDN_URL environment variable from the deploy_admin CI job
• Adds a validation step that confirms the build output:
  • Uses relative asset paths (./assets/)
  • Includes the ghost-cdn-url meta tag (added by PR 1)

Dependencies

PR	Status
PR 1: Ghost #26555 — meta tag + bridge script	Must be merged first
PR 2: Ghost-Moya #135 — sed-based deploy	Must be deployed first

Merge order: PR 1 → PR 2 → this PR. If this merges before PR 2 is deployed, the deploy workflow would receive a relative-path artifact but still try the old Node.js replacement — which would silently produce broken staging builds.

Impact

• Ghost(Pro): Admin artifacts switch from absolute CDN URLs to relative paths. Ghost-Moya's deploy workflow (PR 2) detects the format and applies sed rewriting.
• Self-hosted: No change. GHOST_CDN_URL remains a supported build-time option in vite.config.ts — only the CI environment variable is removed.

Test plan

• CI build produces apps/admin/dist/index.html with relative paths
• Validation step passes (relative paths present, ghost-cdn-url meta present)
• After deploy via Ghost-Moya, staging admin loads assets from staging CDN
• After deploy via Ghost-Moya, production admin loads assets from production CDN
• Self-hosted builds with GHOST_CDN_URL still work as before

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.13%. Comparing base (63703ac) to head (92faa69).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26556      +/-   ##
==========================================
- Coverage   73.14%   73.13%   -0.01%     
==========================================
  Files        1528     1528              
  Lines      120154   120156       +2     
  Branches    14506    14504       -2     
==========================================
- Hits        87886    87879       -7     
- Misses      31247    31272      +25     
+ Partials     1021     1005      -16     

Flag	Coverage Δ
admin-tests	53.74% <ø> (+0.01%)	⬆️
e2e-tests	73.13% <ø> (-0.01%)	⬇️

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[rob-ghost]

Closing — no longer needed. PR #26546 introduces a separate admin-build-cd artifact with relative paths for cd.yml, so the existing CI build still needs GHOST_CDN_URL for the deploy-admin.yml flow. The two deploy paths are now decoupled.