WASM decider: expose the sim host as an MCP tool over mcp-nats

[yordis]

From the WASM decider roadmap (formerly WASM_TODO.md, now retired). Unscheduled idea.

Expose trogon-decider-sim (load/test/simulate compiled decider components, YAML given/when/then scenarios) as an MCP tool via mcp-nats, so agents can load, test, and simulate deciders over the wire.

[yordis]

Placement note from the decider crate-family audit, so this issue carries its own layering context.

Exposing the sim host as an MCP tool straddles L3 and L4: the capability is trogon-decider-sim's (L3), but the thing that ships is a tool surface alongside cli/trogon-decider-test (L4), reached over mcp-nats. Worth deciding early which crate owns the tool definition, because putting it in the sim crate would pull an MCP dependency into a crate whose current dependency set is domain, guest SDK, WASM runtime, and WIT.

Nothing in the current stack blocks it.