ci: add validation workflow + auto-merge stub

HTML validation, JS syntax check, JSON validation, file size budget.
Auto-merge handled by nexus-merge-bot.sh until Gitea runner is set up.

.gitea/workflows/auto-merge.yml

@@ -0,0 +1,10 @@
+# Placeholder — auto-merge is handled by nexus-merge-bot.sh
+# Gitea Actions requires a runner to be registered.
+# When a runner is available, this can replace the bot.
+name: stub
+on: workflow_dispatch
+jobs:
+  noop:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo "See nexus-merge-bot.sh"

.gitea/workflows/ci.yml

@@ -0,0 +1,78 @@
+name: CI
+
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Validate HTML
+        run: |
+          # Check index.html exists and is valid-ish
+          test -f index.html || { echo "ERROR: index.html missing"; exit 1; }
+          # Check for unclosed tags (basic)
+          python3 -c "
+          import html.parser, sys
+          class V(html.parser.HTMLParser):
+              def __init__(self):
+                  super().__init__()
+                  self.errors = []
+              def handle_starttag(self, tag, attrs): pass
+              def handle_endtag(self, tag): pass
+          v = V()
+          try:
+              v.feed(open('index.html').read())
+              print('HTML: OK')
+          except Exception as e:
+              print(f'HTML: FAIL - {e}')
+              sys.exit(1)
+          "
+
+      - name: Validate JavaScript
+        run: |
+          # Syntax check all JS files
+          FAIL=0
+          for f in $(find . -name '*.js' -not -path './node_modules/*' -not -name 'sw.js'); do
+            if ! node --check "$f" 2>/dev/null; then
+              echo "FAIL: $f"
+              FAIL=1
+            else
+              echo "OK: $f"
+            fi
+          done
+          exit $FAIL
+
+      - name: Validate JSON
+        run: |
+          # Check all JSON files parse
+          FAIL=0
+          for f in $(find . -name '*.json' -not -path './node_modules/*'); do
+            if ! python3 -c "import json; json.load(open('$f'))"; then
+              echo "FAIL: $f"
+              FAIL=1
+            else
+              echo "OK: $f"
+            fi
+          done
+          exit $FAIL
+
+      - name: Check file size budget
+        run: |
+          # Performance budget: no single JS file > 500KB
+          FAIL=0
+          for f in $(find . -name '*.js' -not -path './node_modules/*'); do
+            SIZE=$(wc -c < "$f")
+            if [ "$SIZE" -gt 512000 ]; then
+              echo "FAIL: $f is ${SIZE} bytes (budget: 512000)"
+              FAIL=1
+            else
+              echo "OK: $f (${SIZE} bytes)"
+            fi
+          done
+          exit $FAIL