ci: sync translations only for PRs merged into main

The sync workflows triggered on `pull_request: types: [closed]` with no
`branches:` filter, so a PR merged into any branch fired them. A merge into
the long-lived `jb2` branch opened translation PRs in every target repo for
content the published edition does not carry. Mirrors the template change
in QuantEcon/action-translation#308 (v0.28.1).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: mmcky

.github/workflows/sync-translations-fa.yml

@@ -6,6 +6,10 @@ name: Sync Translations (Farsi)
 on:
   pull_request:
     types: [closed]
+    # Only merges into main sync. Without this, a PR merged into any branch
+    # (a long-lived migration branch, say) opens translation PRs for content
+    # the published edition does not carry.
+    branches: [main]
     paths:
       - 'lectures/**/*.md'
       - 'lectures/_toc.yml'

.github/workflows/sync-translations-fr.yml

@@ -6,6 +6,10 @@ name: Sync Translations (French)
 on:
   pull_request:
     types: [closed]
+    # Only merges into main sync. Without this, a PR merged into any branch
+    # (a long-lived migration branch, say) opens translation PRs for content
+    # the published edition does not carry.
+    branches: [main]
     paths:
       - 'lectures/**/*.md'
       - 'lectures/_toc.yml'

.github/workflows/sync-translations-zh-cn.yml

@@ -6,6 +6,10 @@ name: Sync Translations (Simplified Chinese)
 on:
   pull_request:
     types: [closed]
+    # Only merges into main sync. Without this, a PR merged into any branch
+    # (a long-lived migration branch, say) opens translation PRs for content
+    # the published edition does not carry.
+    branches: [main]
     paths:
       - 'lectures/**/*.md'
       - 'lectures/_toc.yml'