init: git-pages init job, ADR 0004, docs, remove PUT fallback

- ADR 0004: commit-status-periaate (API vain tarvittaessa, natiivi riittää)
- git-pages/docs/implementation-notes.md: tekniset huomiot
- init-job.yaml: post-install hook luo placeholderin (.index olemassa)
- values.yaml: initJob.enabled default
- publish-git-pages.sh: poistettu PUT-fallback (init job hoitaa)
- README: maininta automaattisesta initistä

git-pages/values.yaml

@@ -37,6 +37,15 @@ ingress:
 certificate:
   enabled: true
 
+# Post-install init job: creates placeholder site so .index exists.
+# Consumers can use PATCH directly without PUT fallback.
+initJob:
+  enabled: true
+  image:
+    repository: debian
+    tag: bookworm-slim
+    pullPolicy: IfNotPresent
+
 # Optional Helm-managed secret — prefer manual create (see docs/secrets.md).
 publishAuth:
   create: false