feat: add LLM Hello World workflow (Phase 0a)

Tracer bullet for LLM-powered PR review using GitHub Models API.

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

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

.github/workflows/llm-hello-world.yaml

@@ -0,0 +1,147 @@
+name: LLM Hello World
+
+on:
+  workflow_dispatch:
+    inputs:
+      pr_number:
+        description: "PR number to review"
+        required: true
+        type: number
+      phase:
+        description: "Which phase to run"
+        required: true
+        type: choice
+        options:
+          - hello-world
+          - token-probe
+          - triage-poc
+        default: hello-world
+
+env:
+  LLM_ENDPOINT: "https://models.github.ai/inference/chat/completions"
+  LLM_MODEL: "openai/gpt-4.1"
+
+permissions:
+  contents: read
+  pull-requests: write
+  models: read
+
+jobs:
+  hello-world:
+    if: ${{ inputs.phase == 'hello-world' }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Fetch PR details
+        id: pr
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          echo "Fetching PR #${{ inputs.pr_number }}..."
+
+          # Fetch PR title and body
+          PR_JSON=$(gh pr view ${{ inputs.pr_number }} --json title,body,author)
+
+          if [ $? -ne 0 ]; then
+            echo "::error::Failed to fetch PR #${{ inputs.pr_number }}. Does it exist?"
+            exit 1
+          fi
+
+          PR_TITLE=$(echo "$PR_JSON" | jq -r '.title')
+          PR_BODY=$(echo "$PR_JSON" | jq -r '.body // "No description provided"')
+          PR_AUTHOR=$(echo "$PR_JSON" | jq -r '.author.login')
+
+          echo "PR Title: $PR_TITLE"
+          echo "PR Author: $PR_AUTHOR"
+
+          # Store for next step (escape newlines for multiline)
+          {
+            echo "title<<EOF"
+            echo "$PR_TITLE"
+            echo "EOF"
+          } >> $GITHUB_OUTPUT
+
+          {
+            echo "body<<EOF"
+            echo "$PR_BODY"
+            echo "EOF"
+          } >> $GITHUB_OUTPUT
+
+          echo "author=$PR_AUTHOR" >> $GITHUB_OUTPUT
+
+      - name: Call LLM for summary
+        id: llm
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          echo "Calling LLM to summarize PR..."
+
+          # Build the prompt
+          PROMPT="Summarize this PR in one sentence.
+
+          Title: ${{ steps.pr.outputs.title }}
+
+          Description:
+          ${{ steps.pr.outputs.body }}"
+
+          # Escape for JSON
+          PROMPT_ESCAPED=$(echo "$PROMPT" | jq -Rs .)
+
+          # Call the LLM
+          RESPONSE=$(curl -s -w "\n%{http_code}" -X POST "$LLM_ENDPOINT" \
+            -H "Authorization: Bearer $GITHUB_TOKEN" \
+            -H "Content-Type: application/json" \
+            -d "{
+              \"model\": \"$LLM_MODEL\",
+              \"messages\": [
+                {\"role\": \"system\", \"content\": \"You are a helpful assistant that summarizes pull requests concisely.\"},
+                {\"role\": \"user\", \"content\": $PROMPT_ESCAPED}
+              ],
+              \"max_tokens\": 150
+            }")
+
+          # Extract HTTP status code (last line)
+          HTTP_CODE=$(echo "$RESPONSE" | tail -n1)
+          BODY=$(echo "$RESPONSE" | sed '$d')
+
+          echo "HTTP Status: $HTTP_CODE"
+
+          if [ "$HTTP_CODE" != "200" ]; then
+            echo "::error::LLM API call failed with status $HTTP_CODE"
+            echo "Response: $BODY"
+            exit 1
+          fi
+
+          # Extract the summary from the response
+          SUMMARY=$(echo "$BODY" | jq -r '.choices[0].message.content')
+
+          echo "LLM Summary: $SUMMARY"
+
+          {
+            echo "summary<<EOF"
+            echo "$SUMMARY"
+            echo "EOF"
+          } >> $GITHUB_OUTPUT
+
+      - name: Post comment to PR
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          echo "Posting comment to PR #${{ inputs.pr_number }}..."
+
+          COMMENT="## LLM Hello World Test
+
+          **Model:** \`$LLM_MODEL\`
+          **PR Author:** @${{ steps.pr.outputs.author }}
+
+          ### Summary
+          ${{ steps.llm.outputs.summary }}
+
+          ---
+          *This comment was generated by the LLM Hello World workflow (Phase 0a)*"
+
+          gh pr comment ${{ inputs.pr_number }} --body "$COMMENT"
+
+          echo "Comment posted successfully!"