ci: treat the plugin manifest schema as a Go change

plugins/manifest.go:37 declares manifest-schema.json as the input to a real
go:generate directive producing manifest_gen.go, so a schema-only edit skipped
the very check that catches a forgotten regeneration. Verified by adding a
property to the schema and re-running go generate, which changed the output.

All nine commits that ever touched the schema were already caught, but that is
the check working rather than evidence it is unnecessary: an author who forgets
to regenerate cannot reach merged history while the check runs.

This is the only gap of its kind. The repo has two go:generate directives, and
wire's inputs are all Go source.

.github/workflows/detect-changes.sh

@@ -3,7 +3,8 @@
 # Emits per-area change flags to $GITHUB_OUTPUT so the pipeline can skip work a
 # pull request cannot affect:
 #
-#   go    - Go sources, module files, linter config, embedded resources
+#   go    - Go sources, module files, linter config, embedded resources and
+#           the go:generate inputs whose generated output CI verifies
 #   js    - anything under ui/
 #   i18n  - translation files and their validation script
 #   build - anything that ends up in a binary, image or package (i.e. every
@@ -23,7 +24,7 @@
 set -uo pipefail
 export LC_ALL=C
 
-GO_RE='(\.go$|(^|/)go\.(mod|sum)$|^Makefile$|^\.golangci\.yml$|^resources/|^db/migrations/|^tests/|^\.github/workflows/(pipeline\.yml|detect-changes\.sh)$)'
+GO_RE='(\.go$|(^|/)go\.(mod|sum)$|^Makefile$|^\.golangci\.yml$|^resources/|^db/migrations/|^tests/|^plugins/manifest-schema\.json$|^\.github/workflows/(pipeline\.yml|detect-changes\.sh)$)'
 JS_RE='(^ui/|^\.github/workflows/(pipeline\.yml|detect-changes\.sh)$)'
 I18N_RE='(^resources/i18n/|^ui/src/i18n/en\.json$|^\.github/workflows/validate-translations\.sh$|^\.github/workflows/(pipeline\.yml|detect-changes\.sh)$)'
 DOC_ONLY_RE='(\.md$|^LICENSE$|^\.git-blame-ignore-revs$|^\.gitignore$|^\.devcontainer/)'