docs(adr): write ADR-008 — vector store selection for docs-agent

Context

• PR #58 merged;
  a Feast POC (store_via_feast()) alongside the existing store_milvus() path. Both co-exist today but only pymilvus is being actively used. The decision is implicitly made but never documented.

Problem

Every pipeline issue downstream assumes one canonical storage path. Without a written ADR, contributors must guess which implementation to build on — creating risk of divergence and wasted work.

Deliverable

Write docs/adr/ADR-008-vector-store-selection.md covering:

• Context: two patterns after PR #58
• Decision: MilvusClient/pymilvus as the canonical path
• Rationale: proven, lighter deps, thread-safe, no GA blocker
• Future path: Feast when feast[milvus] reaches GA
• Mark kagent-feast-mcp/pipelines/ as legacy in a comment

Acceptance Criteria

• docs/adr/ADR-008-vector-store-selection.md exists and is merged
• kagent-feast-mcp/pipelines/kubeflow-pipeline.py has a legacy comment
• README.md references ADR-008

Related

• PR #58 — merged Feast POC that created the ambiguity
• Parent: #72
• GSoC discussion: #59

Read PR #58 and inspect the existing store_via_feast() and store_milvus() references. Create docs/adr/ADR-008-vector-store-selection.md with the stated decision, rationale, and future path; add the legacy comment in kagent-feast-mcp/pipelines/kubeflow-pipeline.py and reference ADR-008 from README.md.