Add `chainweaver scorecard` for Execution Quality Signals

[dgenio]

Description

Trace data exists, but users lack a simple health summary command.

Use case

Maintainers need quick quality signals across recent runs.

Proposed solution

• Add chainweaver scorecard over execution traces.
• Show failure rate, p95 step latency, drift count, and retry pressure.
• Support table and JSON output formats.
• Allow filtering by flow name and date window.
• Add one docs page with interpretation guidance.

Alternatives considered

• Use raw trace exports only.
• Build external dashboards first.
• Add ad-hoc scripts in examples.