feat(db): add VectorStore capability aware routing

[ohdearquant]

Body:

• Problem: callers currently have to infer vector backend support for filters, batch search, quantization, updates, orphan sweep, and multi-field storage. ADR-044 says capability introspection and filtered search are needed, while the sqlite-vec capability struct reports unsupported features at crates/khive-db/src/stores/vectors.rs:558.
• Proposed fix: add a backend capability routing layer that callers can use before issuing vector operations, and update retrieval/memory paths to route filtered and maintenance work based on declared capabilities rather than backend-specific assumptions.
• Acceptance criteria: capability checks are covered by unit tests; at least one retrieval path chooses pushdown or fallback based on capabilities; unsupported operations return a stable typed error; docs explain how new backends advertise support.
• Affected crates/ADRs: docs/adr/ADR-044-vector-store-extensions.md:20, docs/adr/ADR-044-vector-store-extensions.md:28, crates/khive-db/src/stores/vectors.rs:558, crates/khive-db/src/stores/vectors.rs:561.

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Source: khive-improvement-survey show — proposed backlog (propose-only synthesis). Dedup: NEW.