[User Story] Update ghost.bu documentation for sysext version updates

[noahwhite]

Description

Update the Butane configuration to include documentation about updating the Alloy sysext image reference when new versions are published from the automated build pipeline.

Acceptance Criteria

• Add comment in ghost.bu explaining the image source and automated build process
• Document the process for updating to new Alloy versions:
  1. New release published in alloy-sysext-build repo
  2. CI builds and uploads to R2
  3. Download new image and generate SHA256
  4. Update ghost.bu with new URL and hash
  5. Apply infrastructure changes via OpenTofu
• Create runbook or add to CLAUDE.md for version update process
• Consider creating a helper script to automate hash updates

File to Update

• /workspace/opentofu/modules/vultr/instance/userdata/ghost.bu (lines 11-16)

Current Configuration

- path: /opt/extensions/alloy/alloy-1.10.2-amd64.raw
  mode: 0644
  contents:
    source: https://ghost-sysext-images.separationofconcerns.dev/alloy-1.10.2-amd64.raw
    verification:
      hash: sha256-feb76c5aa5408c267d59508bd39d322c20d6fce44abd686296f4d0ca87e42671

Dependencies

• GHO-35: R2 integration must be working to verify the update process

[linear]

GHO-36