agents: the send(operator) refusal also covers agents without matrix

An agent with no matrix account has no `mcp__matrix__*` tools, so a
refusal that only says "use the matrix MCP tools" points it at something
it cannot call. `OPERATOR_SEND_REFUSAL` now names matrix for an agent
that has it, and tells one without it that there is no direct channel:
the operator reads its `set_status` line, its turn output on the
dashboard, and its logs. The allow-list refusal, the `send` tool
description, the MCP server instructions and the system prompt say
"over matrix if you have it" to match, and the helper-event bullets go
back to "ask the operator" without naming a channel.

hive-agent-mcp/src/mcp/args.rs

@@ -7,7 +7,8 @@ use rmcp::schemars;
 #[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
 pub struct SendArgs {
     /// Logical agent name to deliver the message to (e.g. `"manager"`,
-    /// `"alice"`). `"operator"` is refused: reach the operator over matrix.
+    /// `"alice"`). `"operator"` is refused: reach the operator over matrix
+    /// if you have it.
     pub to: String,
     /// Message body. Plain text; the broker doesn't parse it.
     pub body: String,