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.

docs/turn-loop/mcp.md

@@ -36,9 +36,11 @@ is opt-in via the P3RM1SS10NS tab.
   `in_reply_to: i64` links the message to a prior id for thread
   rendering. Per-agent `services.hyperhive.agent.allowedRecipients`
   (default: empty = unrestricted) limits which names `send` accepts —
-  useful for sandboxing. The broker refuses `to: "operator"` with an error
-  that points at the matrix MCP tools: agents reach the operator over
-  matrix ([matrix](../tools/matrix.md)).
+  useful for sandboxing. The broker refuses `to: "operator"`. The error
+  points an agent that has the matrix MCP tools at matrix
+  ([matrix](../tools/matrix.md)), and tells one without them that the
+  operator reads its `set_status` line, its turn output on the dashboard,
+  and its logs.
 - `recv` — drain inbox. Always an immediate peek, never blocks. `max`
   (default 1, cap 5) drains up to N rows. `recv` prefixes each returned row
   with `[msg #<id>]` (broker row id; note the highest id seen, then pass