feat(site-publish): scope publication with an artifacts selection

A repository whose artifacts ship on different cadences has no way to
publish one of them. Baseline needs it: every merge to main must put the
catalogue live in under five minutes, while `dist/` is content-addressed
and may only be written by a tag release. Today the action iterates
cfg["artifacts"] unconditionally, so the only lever is deleting the
distributions artifact from site.yaml — which changes the stored
publication contract and drives the route-retirement path.

The new `artifacts:` input names the subset this run builds and
publishes. Selection scopes the build, the immutable preflight, the CORS
reconcile, the S3 sync, and credential resolution. It deliberately does
not scope manifest rendering or the immutable-path history: those stay
whole, so a scoped run can never retire another artifact's route or
delete its bucket contents. An undeclared name fails before the first
bucket is touched; `enabled: false` refuses a selection because
decommissioning is whole-site.

Default is unchanged: no input publishes every declared artifact.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UjQqc4qFmdpAWaYfy2Aypb

scripts/build.py

@@ -5,7 +5,14 @@ import subprocess
 import tempfile
 from pathlib import Path
 
-from utils import EXCLUDE_FILES, env, parse_site_yaml, run, validate_artifact_inputs
+from utils import (
+    EXCLUDE_FILES,
+    env,
+    parse_site_yaml,
+    run,
+    selected_artifacts,
+    validate_artifact_inputs,
+)
 
 
 def build_artifact(site_dir, artifact):
@@ -66,5 +73,5 @@ def cmd_build():
 
     validate_artifact_inputs(site_dir, cfg)
 
-    for artifact in cfg["artifacts"]:
+    for artifact in selected_artifacts(cfg):
         build_artifact(site_dir, artifact)