feat(logger): add ISO 8601 timestamps to log output (closes #97)

[komoreka]

Summary

Adds an ISO 8601 timestamp prefix to every log line emitted by createLogger in packages/smithy/src/utils/logger.ts.

Before:

[dispatch-daemon] Spawned worker el-4w8y for task el-2xyz
[cost-service] No pricing found for model "..."

After:

2026-05-05T08:21:44.123Z [dispatch-daemon] Spawned worker el-4w8y for task el-2xyz
2026-05-05T08:21:44.125Z [cost-service] No pricing found for model "..."

Closes #97.

Changes

• packages/smithy/src/utils/logger.ts: exports a new timestampsEnabled() helper that reads LOG_TIMESTAMPS fresh on each call (same pattern as the existing getLogLevel()). The createLogger factory's inner emit helper uses it to prepend new Date().toISOString() before the [service-name] prefix when enabled.
• packages/smithy/src/utils/logger.bun.test.ts: 14 new tests covering timestamp format, opt-out behavior, extra args pass-through, and all four log levels.
• packages/smithy/src/services/steward-scheduler.test.ts: updated three existing spy assertions to expect the new leading timestamp argument.
• .changeset/logger-timestamps.md: patch bump for @stoneforge/smithy.

Opt-out

Set LOG_TIMESTAMPS=0 to disable the prefix:

LOG_TIMESTAMPS=0 sf serve smithy

This matches the existing LOG_LEVEL env-var pattern and is useful when piping to systemd, journald, or any other log aggregator that adds its own timestamps.

Test plan

• bun test packages/smithy/src/utils/logger.bun.test.ts passes (14/14)
• Full smithy test suite: 1582 pass, 0 fail
• NODE_ENV=development npx turbo run typecheck --filter=@stoneforge/smithy clean