rust(feat): accept host-resolved feature flags in sift-cli mcp

[lineville]

Description

• sift-cli mcp reads SIFT_MCP_FEATURE_FLAGS (a JSON object of flag name to variant, e.g. {"chat-agents-service":"on"}) and skips its startup flag fetch when it is set.
• Sift agent pods already resolve the user's flags before they start sift-cli. When the startup fetch fails, sift-cli hides every flag-gated tool with no error, so the agent loses create_artifact and the test report tools but keeps update_artifact. This was seen in prod.
• Invalid JSON logs a warning and falls back to the fetch. Hosts that do not set the variable behave as before.

Verification

• cargo test -p sift_mcp feature_flags and cargo test -p sift_cli mcp:: pass, including new tests for parsing and for skipping the fetch.
• cargo clippy shows no new warnings.

[lineville]

dont merge me