rust(feat): generic artifact store tools

[evan-sift]

Description

Extends the artifact MCP tools for the generic artifact store exposed by sift.artifacts.v1.

• Syncs the updated artifacts.proto (storage class, created-via surface, kind, JSON payload, metadata, links, CEL filter and order_by, link RPCs) and regenerates the Rust bindings.
• list_artifacts accepts filter (CEL) and order_by, and documents the filterable fields, enum comparisons, links.exists(...) and metadata["<key>"] forms. Results are scoped to the caller's organization.
• create_artifact accepts kind, storage_class (file | structured | blob), created_via (chat | canvas | sdk | upload), a structured JSON payload, metadata, and links. Local validation rejects payload/storage-class mismatches before any request. Omitted container fields stay unset on append so existing artifacts keep their class.
• download_artifact documents that structured artifacts carry their payload inline.
• Mock server covers the three new link RPCs; tests cover the new parameters and rejection paths.

Existing callers of list_artifacts, create_artifact, and download_artifact keep working unchanged.

Depends on the matching server change to sift.artifacts.v1 landing first; against an older server the new create parameters are ignored and filter / order_by are unsupported.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-features -p sift_mcp -p sift_test_util (only the pre-existing too_many_arguments warning in service/report_templates)
• cargo test -p sift_mcp -p sift_test_util: 524 + 6 passed