chore(config): freeze v9 and bump latest to v10

[dgageot]

The configuration schema evolves incrementally as new features are added. To support this safely, we freeze completed schema versions as immutable numbered packages and continue development on latest.

This change creates pkg/config/v9 as a frozen copy of the previous latest package and bumps latest to represent the work-in-progress v10 schema. The upgrade path in pkg/config/versions.go is updated so that v9 configs upgrade through v9's parser before reaching v10.

No behavior changes for existing configurations.

[docker-agent]

Assessment: 🟢 APPROVE

This PR correctly implements the v9 freeze and v10 bump pattern. The version upgrade chain has been reviewed end-to-end:

• pkg/config/v9/types.go declares Version = "9" ✅
• pkg/config/v9/parse.go imports v8 as previous and upgrades v8.Config → v9.Config ✅
• pkg/config/latest/types.go declares Version = "10" ✅
• pkg/config/latest/parse.go imports v9 as previous for the v9.Config → v10.Config step ✅
• pkg/config/versions.go inserts v9.Register(...) between v8 and latest in the upgrade chain ✅
• agent-schema.json description updated to v10 ✅

No bugs found in the added code. The pkg/config/v9/ package is a correct frozen snapshot with all files declaring package v9, proper import paths, and no stale references to the old latest package.