feature: introduce a shared Milvus lifecycle seam across runtime stores

[Xunzhuo]

Describe the feature

Create a shared Milvus lifecycle seam so cache, memory, vectorstore, and replay stores stop reimplementing the same connection/bootstrap/index/load/retry mechanics independently.

Primary files and seams:

• src/semantic-router/pkg/memory/milvus_store.go
• src/semantic-router/pkg/cache/milvus_cache.go
• src/semantic-router/pkg/cache/hybrid_cache.go
• src/semantic-router/pkg/vectorstore/milvus_backend.go
• src/semantic-router/pkg/routerreplay/store/milvus.go
• src/semantic-router/pkg/responsestore/{interface.go,factory.go}

Primary layer

global level

Why this layer?

This is a backend-adapter architecture cleanup shared by multiple runtime packages rather than a single signal/plugin feature.

Why do you need this feature?

The current repo audit confirmed that Milvus lifecycle logic is still duplicated across several packages:

• client setup
• collection bootstrap
• index creation
• collection loading
• retry/flush policy

Those packages have domain-specific query semantics on top, but the lifecycle mechanics are largely the same.

This gap is tracked in docs/agent/tech-debt/td-021-milvus-adapter-duplication-across-runtime-stores.md and in docs/agent/plans/pl-0006-design-elegance-ratchet.md, but there is currently no dedicated GitHub issue for external contributors.

Additional context

Suggested acceptance criteria:

• a shared adapter/helper is used by the main Milvus-backed runtime packages instead of repeating bootstrap logic in each package
• collection creation, index creation, loading, and retry policy are defined once unless a store has a strong domain-specific reason to diverge
• domain package tests focus on schema/query semantics instead of repeatedly mocking the same lifecycle behavior
• validation includes make test-semantic-router and the relevant agent gates for touched files

This is a good help wanted item, but not a good first issue: contributors need to understand multiple runtime store packages and avoid flattening away domain-specific semantics while extracting the shared Milvus seam.

[henschwartz]

/assign