rust(feat): calculated channel MCP tools

[evan-sift]

Description

Adds a calculated_channels domain to the MCP server.

Tool surface: list_calculated_channels, list_calculated_channel_versions, create_calculated_channel, update_calculated_channel, archive_calculated_channel, unarchive_calculated_channel.

Nested channel references ride in expression_channel_references_json, a documented JSON string parameter, keeping every other parameter flat. Update pre-reads the channel before writing: the update mask paths cover whole configuration messages, so a mask-only write would clear sibling fields the caller never mentioned.

Surface note: there is no dedicated archive RPC. The public API archives a calculated channel by setting archived_date through the update mask. archive_calculated_channel and unarchive_calculated_channel are exposed as separate tools anyway, so an agent does not have to know that, and so each can carry a destructive write gate.

Verification

• cargo test -p sift_mcp: 50 new mock-backed tests (pagination, limit clamp, mask construction, archive/unarchive request shape, write gates, JSON parameter validation), full crate green.
• cargo build -p sift_mcp --no-default-features, cargo fmt --all -- --check, cargo test -p sift_cli cmd::agent: clean.
• MCP protocol check against a development org: all six tools registered, live list_calculated_channels round trip, and all four write tools reject with the remediation message when the server runs without --allow-create / --allow-destructive.
• Agent tool-selection gates (list, versions, create-blocked, create, archive) pass against the full stack binary.