fix(ci): fix diff extraction for new files, stderr pollution, and api-key check

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

Author: watney1024

.github/workflows/ai-code-review.yml

@@ -55,18 +55,12 @@ jobs:
               echo "Reviewing: $FILE"
 
               # ── Get per-file diff ──
-              DIFF=$(git diff "origin/$BASE_REF...HEAD" -- "$FILE" 2>&1) || { echo "  [ERROR] git diff failed for $FILE: $DIFF"; continue; }
+              DIFF=$(git diff "origin/$BASE_REF...HEAD" -- "$FILE" 2>/dev/null) || { echo "  [ERROR] git diff failed for $FILE"; continue; }
 
-              # ── If empty, try direct comparison (handles new files / special chars) ──
+              # ── If empty (new file / special chars), show full content ──
               if [ -z "$DIFF" ]; then
-                DIFF=$(git diff "origin/$BASE_REF" HEAD -- "$FILE" 2>&1) || true
-              fi
-
-              # ── If still empty, try showing file content (new file fallback) ──
-              if [ -z "$DIFF" ]; then
-                FILE_CONTENT=$(cat "$FILE" 2>/dev/null) || { echo "  [SKIP] $FILE — cannot read"; continue; }
-                LINE_COUNT=$(echo "$FILE_CONTENT" | wc -l)
-                DIFF="--- /dev/null\n+++ b/$FILE\n@@ -0,0 +1,$LINE_COUNT @@\n$(echo "$FILE_CONTENT" | sed 's/^/+/')"
+                LINE_COUNT=$(wc -l < "$FILE" 2>/dev/null) || { echo "  [SKIP] $FILE — cannot read"; continue; }
+                DIFF="--- /dev/null\n+++ b/$FILE\n@@ -0,0 +1,$LINE_COUNT @@\n$(cat "$FILE" | sed 's/^/+/')"
               fi
 
               # ── Check if binary ──
@@ -94,8 +88,8 @@ jobs:
                 }')
 
               # ── Check API key ──
-              if [ -z "$SENSENOVA_API_KEY" ] || [ "$SENSENOVA_API_KEY" = "sk-placeholder" ]; then
-                echo "  [ERROR] SENSENOVA_API_KEY is not set or is placeholder. Skipping review for $FILE."
+              if [ -z "$SENSENOVA_API_KEY" ] || [[ "$SENSENOVA_API_KEY" != sk-* ]]; then
+                echo "  [ERROR] SENSENOVA_API_KEY is not set or invalid. Skipping review for $FILE."
                 continue
               fi