Context

Implement the full single-agent task lifecycle per spec 6.1. This covers the complete flow from task creation through assignment, execution, and completion, including all status transitions, artifact collection, and proxy overhead metrics.

Acceptance Criteria

Task Lifecycle

• Task status transitions enforced: CREATED → ASSIGNED → IN_PROGRESS → IN_REVIEW → COMPLETED (or BLOCKED/CANCELLED)
• Task assignment: agent receives task with full context and acceptance criteria
• Agent executes task using LLM + tools from the agent engine
• Status transitions validated (no invalid jumps, e.g., CREATED → COMPLETED)
• Artifacts collected during execution (files created, outputs generated)
• Task completion includes summary of work done
• Error handling: task failure with reason, timeout with configurable duration

Proxy Overhead Metrics (§10.5 M3)

• Cost tracked for the entire task lifecycle
• turns_per_task: number of LLM turns to complete a task
• tokens_per_task: total tokens (input + output) consumed
• cost_per_task: total USD cost (derived from existing CostRecord data)
• Metrics available on task completion for analysis

Testing

• Unit tests for status state machine
• Integration test for full lifecycle (assign → execute → complete)

Dependencies

• Depends on Implement agent engine core with ExecutionLoop protocol integration (DESIGN_SPEC §3.1, §6.1, §6.5) #11 (agent engine core)
• Depends on Design and implement basic tool system (registry, invocation, results) #15 (basic tool system)

Design Spec Reference

• §6.1, §6.2 — Task Lifecycle and Workflow
• §10.5 — LLM Call Analytics (M3: proxy overhead metrics)