Add require-label option for on-demand reviews (#13)

## Summary
Adds ability to only run code reviews when a specific label is present
on the PR. The review can be triggered by adding the label, even without
pushing new commits.

## New Input
`require-label` (default: empty)
- Set to a label name (e.g., `needs-review`) to only run when that label
is present
- Leave empty to review all PRs (current behavior)

## Workflow Configuration
To trigger on label addition, include `labeled` in your workflow:

```yaml
on:
  pull_request:
    types: [opened, synchronize, reopened, labeled]

jobs:
  review:
    steps:
      - uses: PSPDFKit-labs/claude-code-review@main
        with:
          claude-api-key: ${{ secrets.ANTHROPIC_API_KEY }}
          require-label: 'needs-review'
```

## Behavior
| Label present | Action |
|---------------|--------|
| Yes | Run review |
| No | Skip with message "required label 'X' not found" |

## Test plan
- [ ] Test PR without label → should skip
- [ ] Add label → should trigger review
- [ ] Test PR with label from start → should run

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: matej

.github/workflows/code-review.yml

@@ -2,6 +2,7 @@ name: Code Review
 
 on:
   pull_request:
+    types: [opened, synchronize, reopened, labeled]
   workflow_dispatch:
 
 jobs:

action.yml

@@ -63,6 +63,11 @@ inputs:
     required: false
     default: 'true'
 
+  require-label:
+    description: 'Only run review if this label is present on the PR. Leave empty to review all PRs. To trigger on label addition, add "labeled" to your workflow pull_request types.'
+    required: false
+    default: ''
+
 outputs:
   findings-count:
     description: 'Number of code review findings'
@@ -106,6 +111,8 @@ runs:
         RUN_EVERY_COMMIT: ${{ inputs.run-every-commit }}
         SKIP_DRAFT_PRS: ${{ inputs.skip-draft-prs }}
         IS_DRAFT: ${{ github.event.pull_request.draft }}
+        REQUIRE_LABEL: ${{ inputs.require-label }}
+        PR_LABELS: ${{ toJSON(github.event.pull_request.labels.*.name) }}
       run: |
         # Check if ClaudeCode should be enabled
         ENABLE_CLAUDECODE="true"
@@ -116,16 +123,26 @@ runs:
           PR_NUMBER="$PR_NUMBER"
           CACHE_HIT="${{ steps.claudecode-history.outputs.cache-hit }}"
 
+          # Check if required label is present
+          if [ -n "$REQUIRE_LABEL" ]; then
+            if echo "$PR_LABELS" | jq -e --arg label "$REQUIRE_LABEL" 'index($label) != null' > /dev/null 2>&1; then
+              echo "Required label '$REQUIRE_LABEL' found on PR #$PR_NUMBER"
+            else
+              echo "Skipping code review: required label '$REQUIRE_LABEL' not found on PR #$PR_NUMBER"
+              ENABLE_CLAUDECODE="false"
+            fi
+          fi
+
           # Skip draft PRs if configured
-          if [ "$SKIP_DRAFT_PRS" == "true" ] && [ "$IS_DRAFT" == "true" ]; then
+          if [ "$ENABLE_CLAUDECODE" == "true" ] && [ "$SKIP_DRAFT_PRS" == "true" ] && [ "$IS_DRAFT" == "true" ]; then
             echo "Skipping code review for draft PR #$PR_NUMBER"
             ENABLE_CLAUDECODE="false"
           # Now check cache - if ClaudeCode has already run, disable unless run-every-commit is true
           # Check if marker file exists (cache may have been restored from a different SHA)
-          elif [ "$RUN_EVERY_COMMIT" != "true" ] && [ -f ".claudecode-marker/marker.json" ]; then
+          elif [ "$ENABLE_CLAUDECODE" == "true" ] && [ "$RUN_EVERY_COMMIT" != "true" ] && [ -f ".claudecode-marker/marker.json" ]; then
             echo "ClaudeCode has already run on PR #$PR_NUMBER (found marker file), forcing disable to avoid false positives"
             ENABLE_CLAUDECODE="false"
-          elif [ "$RUN_EVERY_COMMIT" == "true" ] && [ -f ".claudecode-marker/marker.json" ]; then
+          elif [ "$ENABLE_CLAUDECODE" == "true" ] && [ "$RUN_EVERY_COMMIT" == "true" ] && [ -f ".claudecode-marker/marker.json" ]; then
             echo "ClaudeCode has already run on PR #$PR_NUMBER but run-every-commit is enabled, running again"
           elif [ "$ENABLE_CLAUDECODE" == "true" ]; then
             echo "ClaudeCode will run for PR #$PR_NUMBER (first run)"