[Metrics] Export CI reliability metrics as JSON and CSV

[Dyu20705]

Problem statement

RunSift can become more valuable when it summarizes patterns across workflow runs, not only a single failure.

Implementation notes

Compute failure frequency, flaky job candidates, duration trends, category distribution, and MTTR placeholder from stored run metadata. Export results as JSON and CSV.

Checklist

• Define metrics schema
• Compute failure frequency
• Identify flaky job candidates
• Compute duration trends
• Compute failure category distribution
• Add MTTR placeholder
• Export JSON and CSV

Acceptance criteria

• Metrics can be generated from fixture metadata
• JSON and CSV outputs are documented
• Heuristic metrics are clearly labeled
• Exports are suitable for future dashboard work

Epic

#19