ci: serialize main pushes with shared concurrency group

Prevents race between build_toc and update-tokens workflows that both
push to main on merge; with a shared group the second run waits and
checks out fresh main, avoiding the commit/push conflict.

https://claude.ai/code/session_01Mm5cao4DWuqXogHPaccCTy

Author: claude

.github/workflows/build-toc.yaml

@@ -16,6 +16,10 @@ on:
     branches:
       - main
 
+concurrency:
+  group: main-push
+  cancel-in-progress: false
+
 jobs:
   build_toc:
     # Only run on PR merge or direct push to main

.github/workflows/update-token-count.yml

@@ -12,6 +12,10 @@ on:
 permissions:
   contents: write
 
+concurrency:
+  group: main-push
+  cancel-in-progress: false
+
 jobs:
   update-tokens:
     runs-on: ubuntu-latest