feat(HAI-113): add structured logger and replace console calls

- Create logger module with log levels and structured output (packages/engine/src/logger.ts)
- Replace console.log/warn/error calls with structured logger across engine modules
- Export logger from engine package index
- Add comprehensive tests for logger functionality
- Clean up CI/release workflows and update docs

packages/engine/src/logger.ts

@@ -0,0 +1,63 @@
+/**
+ * Lightweight structured logger for the `@hai/engine` package.
+ *
+ * Usage:
+ * ```ts
+ * import { createLogger } from "./logger.js";
+ * const log = createLogger("my-module");
+ * log.log("hello");   // → console.log("[my-module] hello")
+ * log.warn("oops");   // → console.warn("[my-module] oops")
+ * log.error("fail");  // → console.error("[my-module] fail")
+ * ```
+ *
+ * All engine subsystems should use the pre-built instances exported below
+ * rather than calling `console.*` directly. This gives us a single point
+ * of control for filtering, suppressing (e.g. in tests), or redirecting
+ * engine log output in the future.
+ */
+
+export interface Logger {
+  log(message: string, ...args: unknown[]): void;
+  warn(message: string, ...args: unknown[]): void;
+  error(message: string, ...args: unknown[]): void;
+}
+
+/**
+ * Create a structured logger that prefixes every message with `[prefix]`.
+ *
+ * @param prefix - Short subsystem name, e.g. `"scheduler"` or `"executor"`.
+ * @returns A `Logger` whose `log`, `warn`, and `error` methods delegate to
+ *          the corresponding `console` method with the prefix prepended.
+ */
+export function createLogger(prefix: string): Logger {
+  const tag = `[${prefix}]`;
+  return {
+    log(message: string, ...args: unknown[]) {
+      console.log(`${tag} ${message}`, ...args);
+    },
+    warn(message: string, ...args: unknown[]) {
+      console.warn(`${tag} ${message}`, ...args);
+    },
+    error(message: string, ...args: unknown[]) {
+      console.error(`${tag} ${message}`, ...args);
+    },
+  };
+}
+
+/** Logger for the scheduler subsystem. */
+export const schedulerLog = createLogger("scheduler");
+
+/** Logger for the task executor subsystem. */
+export const executorLog = createLogger("executor");
+
+/** Logger for the triage processor subsystem. */
+export const triageLog = createLogger("triage");
+
+/** Logger for the merge/auto-merge subsystem. */
+export const mergerLog = createLogger("merger");
+
+/** Logger for the worktree pool subsystem. */
+export const worktreePoolLog = createLogger("worktree-pool");
+
+/** Logger for the review subsystem. */
+export const reviewerLog = createLogger("reviewer");