feat: delete previous review before posting new one (#6)

Before posting a review, the bot now:
1. Calls GET /api/v1/user to identify its own login
2. Lists all reviews on the PR
3. Deletes any existing reviews from itself
4. Posts the fresh review

This keeps PR threads clean — one review per bot at any time.

New Gitea client methods:
- GetAuthenticatedUser() — token self-identification
- ListReviews() — fetch reviews on a PR
- DeleteReview() — delete a review by ID

Flag: --update-existing / UPDATE_EXISTING (default true)
Set to false to preserve old behavior (stack reviews).

All delete failures are non-fatal (logged as warnings).

Closes #6

.gitea/actions/review/action.yml

@@ -66,6 +66,10 @@ inputs:
     description: 'Print review to stdout instead of posting'
     required: false
     default: 'false'
+  update-existing:
+    description: 'Delete previous review from same bot before posting (default true)'
+    required: false
+    default: 'true'
 
 runs:
   using: 'composite'
@@ -145,6 +149,7 @@ runs:
         LLM_TEMPERATURE: ${{ inputs.temperature }}
         LLM_TIMEOUT: ${{ inputs.timeout }}
         LLM_PROVIDER: ${{ inputs.llm-provider }}
+        UPDATE_EXISTING: ${{ inputs.update-existing }}
       run: |
         ARGS=""
         if [ "${{ inputs.dry-run }}" = "true" ]; then