ci: add workflow_dispatch trigger to image workflows

Allows on-demand image builds from any branch via gh workflow run,
producing :sha-<short> tags for downstream integration testing
without widening the push-trigger branches list. Applied to both
indexer-agent-image.yml and indexer-cli-image.yml for parity.

Author: RembrandtK

.github/workflows/indexer-agent-image.yml

@@ -1,6 +1,7 @@
 name: Indexer Agent Image
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - main

.github/workflows/indexer-cli-image.yml

@@ -1,6 +1,7 @@
 name: Indexer CLI Image
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - main