fix(hooks): read session_id from stdin JSON instead of env var

PreToolUse hooks receive context via stdin as JSON, not as environment
variables. Parse session_id with jq from the hook input payload.

Author: delexw

.claude/hooks/pre-commit.sh

@@ -4,6 +4,10 @@
 
 set -uo pipefail
 
+HOOK_INPUT=$(cat)
+SESSION_ID=$(printf '%s' "$HOOK_INPUT" | jq -r '.session_id // ""')
+FLAG_FILE="/tmp/claude-tests-confirmed${SESSION_ID:+-$SESSION_ID}"
+
 cd "$CLAUDE_PROJECT_DIR"
 
 STAGED=$(git diff --cached --name-only)
@@ -12,9 +16,6 @@ if [ -z "$STAGED" ]; then
   exit 0
 fi
 
-SESSION_ID="${CLAUDE_CODE_SESSION_ID:-${CLAUDE_SESSION_ID:-}}"
-FLAG_FILE="/tmp/claude-tests-confirmed${SESSION_ID:+-$SESSION_ID}"
-
 ERRORS=""
 
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