docs: matrix MCP per-agent option + tool surface (follow-up to #603)

README.md

@@ -145,6 +145,17 @@ the container as environment variables. Common options:
   replace the whole dist. Paths with leading `/` or `..` segments are
   rejected at eval time.
 
+- **`hyperhive.matrix.enable`** — Boolean (default `true`). When true,
+  each agent container runs `hive-matrix-daemon` (a long-running
+  matrix-sdk process that holds the per-agent client + sync) and
+  auto-injects `hive-matrix-mcp` as a stdio MCP server so claude can
+  call the matrix tools (`mcp__matrix__send_message`, `send_dm`,
+  `send_reaction`, `send_reply`, `mark_read`, `list_rooms`,
+  `list_room_members`, `read_room`). Silently no-ops when
+  `<state>/matrix-token` is absent (i.e., the host-level
+  `hyperhive.matrix` tuwunel container hasn't provisioned the account
+  yet). Set to `false` to opt a specific agent out of matrix.
+
 See `nix/templates/harness-base.nix` for the full list of options and
 their descriptions.