feat: add structured event-driven logger

Level-based logging (debug, info, warn, error) with:
- Colorized console output for headless mode
- EventEmitter for TUI consumption
- Quiet mode for terminal dashboard
- ISO timestamps and JSON-compatible metadata

Zero dependencies — pure Node.js EventEmitter.

src/log.js

@@ -0,0 +1,51 @@
+// Structured logger with event emission for TUI consumption.
+// Levels: debug < info < warn < error
+// In headless mode, writes to stderr (keeps stdout clean for piping).
+// In TUI mode, emits events consumed by the dashboard.
+
+import { EventEmitter } from 'node:events';
+
+const LEVELS = { debug: 0, info: 1, warn: 2, error: 3 };
+const COLORS = { debug: '\x1b[90m', info: '\x1b[36m', warn: '\x1b[33m', error: '\x1b[31m' };
+const RESET = '\x1b[0m';
+const ICONS = { debug: '·', info: '●', warn: '⚠', error: '✗' };
+
+class Logger extends EventEmitter {
+  #level = LEVELS.info;
+  #quiet = false;
+
+  setLevel(level) {
+    this.#level = LEVELS[level] ?? LEVELS.info;
+  }
+
+  /** Suppress console output (TUI mode — events only). */
+  quiet(on = true) {
+    this.#quiet = on;
+  }
+
+  #log(level, msg, meta) {
+    if (LEVELS[level] < this.#level) return;
+    const ts = new Date();
+    const entry = {
+      ts,
+      time: ts.toLocaleTimeString('en-GB', { hour12: false }),
+      level,
+      icon: ICONS[level],
+      msg,
+      meta,
+    };
+    this.emit('entry', entry);
+    if (!this.#quiet) {
+      const prefix = `${COLORS[level]}${ICONS[level]}${RESET}`;
+      const metaStr = meta ? ` ${COLORS.debug}${JSON.stringify(meta)}${RESET}` : '';
+      process.stderr.write(`  ${entry.time}  ${prefix} ${msg}${metaStr}\n`);
+    }
+  }
+
+  debug(msg, meta) { this.#log('debug', msg, meta); }
+  info(msg, meta)  { this.#log('info', msg, meta); }
+  warn(msg, meta)  { this.#log('warn', msg, meta); }
+  error(msg, meta) { this.#log('error', msg, meta); }
+}
+
+export const log = new Logger();