Update task.md with Phase 1 completion status

[Claude]

Marked all Phase 1 (v0.3.0) items as completed in the root task.md implementation plan.

Changes

• Phase 1 features completed:
  • fs init command with five-layer directory structure
  • internal/riskgate/ pre-trade risk engine with CheckOrder validation
  • Controller integration with preTradeCheck in broker.ProcessLedger
  • Risk configuration files (policy.json, pre_trade.json, position_limits.json)
  • ORDER extended fields (source, signal_refs, rebalance_id)
  • Audit log skeleton in internal/audit/
  • Documentation updates

All Phase 1 checkboxes changed from [ ] to [x] to reflect implementation completion.