rust(feat): artifact tools for the generic artifact store

[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, JSON payload, metadata, versioned links, CEL filter and order_by) and regenerates the Rust bindings. The public proto carries the stable surface; the link and discovery RPCs stay internal for now.
• 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 storage_class (file | structured | blob), created_via (agent | canvas | upload), a structured JSON payload, metadata, and links. created_via is required by the server and defaults to agent, since the tool runs inside agent sessions. Local validation rejects payload/storage-class mismatches before any request. storage_class stays unset on append so existing artifacts keep their class.
• download_artifact documents that structured artifacts carry their payload inline.
• 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.

Verification

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