createLogger: migrate to pluggable log adapters (stdout + file + custom sinks) #1238

Motivation

packages/utils/src/logger.ts (createLogger) is env-driven and picks one implicit sink: if AGENT_RELAY_LOG_FILE is set it writes to the file and returns, otherwise it writes to the console. Fleet-node structured logging (#1237 — capabilities registered, actions invoked/completed/failed) now flows through it, which surfaces the limitations:

• No simultaneous sinks. You cannot tee to both a file and stdout — the file path short-circuits console output entirely (logger.ts ~L90-102).
• No pluggable adapters. Format (text vs JSON) and destination are hardcoded and chosen by env vars; there's no way to register a custom sink (ship to relaycast, PostHog, syslog, an in-memory ring buffer for tests).
• Process-global config. Level/file/format are read from process.env per call, so every component (broker, router, connection, fleet) shares one destination — you can't route fleet to its own file while leaving broker logs on the console.
• Blocking writes. appendFileSync per line; no buffering/rotation.

Proposal

Introduce an adapter interface and let a logger fan out to N adapters:

interface LogAdapter {
  write(entry: LogEntry): void;      // or Promise<void> for async sinks
  minLevel?: LogLevel;               // per-adapter filtering
}

createLogger('fleet', { adapters: [stdoutAdapter(), fileAdapter('./node.log', { json: true })] })

• Ship built-in stdoutAdapter / stderrAdapter / fileAdapter (with JSON option, and optional rotation/buffering).
• Keep the AGENT_RELAY_LOG_* env vars as a default adapter set for back-compat so existing callers (and the relay node up --log-* flags added in Fleet node structured logging: capabilities registered and actions invoked #1237) keep working.
• Per-component adapter overrides so fleet logs can go somewhere distinct from broker/router logs.

Context

Surfaced while adding fleet-node structured logging (#1237). That change deliberately reused createLogger and injects it into serveNode via a FleetLogger seam, so once adapters land, fleet gains multi-sink/custom-sink output with no changes on the fleet side.