ci: restore manual plugin deploys

[raymondjacobson]

Summary

• restore manual Main Deploy runs with optional commit_sha and service inputs
• keep push-to-main deploys changed-app-only while inlining the build/multiarch/publish path into deploy.yml
• remove the unused PR build workflow and reusable Build Services workflow
• update Docker build docs for the new manual deploy behavior

CI cleanup evidence

• stale Build Docker Images workflow .github/workflows/build-apps.yml last ran on 2025-09-03 and is disabled
• stale Build Docker Images workflow .github/workflows/build.yml last ran on 2025-09-04 and is disabled
• Build Services had no direct runs; it was only reusable plumbing for deploy/PR build

Verification

• git diff --check
• go run github.com/rhysd/actionlint/cmd/actionlint@latest