feat: Add concurrency limits to trigger workflows

Add max 3 concurrent run limits:
- GitHub Actions: concurrency groups prevent workflow queue buildup
- trigger-server: tracks concurrent runs, rejects with 429 if at max
- Configurable via MAX_CONCURRENT env var (defaults to 3)
- Returns running count and max in trigger response

This prevents resource exhaustion when workflows trigger frequently.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.github/workflows/improve.yml

@@ -5,6 +5,10 @@ on:
     - cron: '*/30 * * * *'
   workflow_dispatch:
 
+concurrency:
+  group: improve-sprite-trigger
+  cancel-in-progress: false
+
 jobs:
   trigger:
     runs-on: ubuntu-latest

.github/workflows/refactor.yml

@@ -7,6 +7,10 @@ on:
     types: [opened, reopened]
   workflow_dispatch:
 
+concurrency:
+  group: refactor-sprite-trigger
+  cancel-in-progress: false
+
 jobs:
   trigger:
     runs-on: ubuntu-latest