### Motivation
This PR evolves the harness from a pure execution runtime into an
**observable, replayable agent evaluation platform**. The current
`harness/graph` checkpoint mechanism is insufficient for true
event-sourced introspection—we need append-only event logs capturing
every tool call, state transition, memory write, and approval decision,
enabling deterministic replay, fork/diff, postmortem analysis, and
time-travel debugging.
### Key Design Goals
1. **Event-Sourced Execution Model**
Replace coarse checkpoints with granular, append-only event logs. Every
operation becomes a durable event: tool invocation, state mutation,
memory update, human approval. This unlocks deterministic replay,
branching execution histories, and regression datasets derived directly
from production failures.
2. **First-Class Replay & Evaluation Loop**
Replay is not an afterthought—it is a core primitive. A single live run
seeds an offline corpus that supports: repeated playback, model
substitution, tool result mocking, and strategy comparison. The harness
graduates from "executor" to "continuous evaluation platform" where
failed production traces convert directly into offline regression
suites.
3. **Operational Observability**
Beyond raw traces, expose metrics that prove stability over time:
- Tool success / failure rates
- Approval latency distributions
- Retry frequencies
- Checkpoint restore reliability
- Memory retrieval quality
- Cost per completed task
- Fork replay pass rates
The underlying thesis: the bottleneck for most agent systems is not
execution capability, but the inability to **demonstrate continuous,
measurable improvement**.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
