Implement structured logging with pino (P2.17)

Add pino-based logger module for production observability:
- JSON output to stdout for log aggregators (Loki, ELK)
- Configurable via LOG_LEVEL environment variable (defaults to "info")
- Log levels: error, warn, info, debug
- Error objects serialized with type, message, and stack trace
- Child logger support for bound context
- ISO 8601 timestamps in all log entries

Test coverage: 16 tests covering JSON format, log levels, error
serialization, and child loggers.

Total tests now: 553 passing across 31 test files.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/lib/logger.ts

@@ -0,0 +1,26 @@
+// ABOUTME: Structured logging module using pino for JSON output to stdout.
+// ABOUTME: Configurable via LOG_LEVEL env var, outputs parseable logs for aggregators.
+
+import pino from "pino";
+
+/**
+ * PhaseFlow logger configured per observability spec.
+ *
+ * Log levels: error, warn, info, debug
+ * Output: JSON to stdout
+ * Configuration: LOG_LEVEL env var (defaults to "info")
+ *
+ * Usage:
+ *   logger.info({ userId: "123" }, "User logged in");
+ *   logger.error({ err: error, userId: "123" }, "Operation failed");
+ */
+export const logger = pino({
+  level: process.env.LOG_LEVEL || "info",
+  formatters: {
+    level: (label) => ({ level: label }),
+  },
+  timestamp: () => `,"timestamp":"${new Date().toISOString()}"`,
+  messageKey: "message",
+});
+
+export type Logger = typeof logger;