Add Insight bounded diagnostic retention

[josiahking]

Summary

• add explicit count-bounded retention to Insight diagnostic stores
• prune oldest stored diagnostic batches deterministically before unique inserts
• preserve duplicate safety and validate SQLite snapshots before pruning
• handle existing over-capacity SQLite stores on the next successful unique save
• document retention behavior and keep time-based retention out of scope

Validation

• focused in-memory and SQLite retention tests passed
• full Insight suite passed with SQLite enabled
• root PHPUnit, PHPStan, dependency architecture and style checks passed
• release package, supply-chain, deterministic split, prerelease consumer and application skeleton validation passed