docs: rename OACP to Harness Protocol and revamp protocol.org with verbose literate text

tests/org-agent-test.el

@@ -15,7 +15,7 @@
       (should (string= "000014(:type :EVENT :id 1)" captured-framed)))))
 
 (ert-deftest test-org-agent-parsing ()
-  "Verify that the filter correctly parses OACP framed messages."
+  "Verify that the filter correctly parses Harness Protocol framed messages."
   (let ((mock-buffer (generate-new-buffer " *org-agent-test*"))
         (received-plist nil))
     (cl-letf (((symbol-function 'org-agent--handle-message)

tests/protocol-tests.lisp

@@ -2,9 +2,9 @@
   (:use :cl :fiveam :org-agent))
 (in-package :org-agent-tests)
 
-(def-suite oacp-suite
-  :description "Test suite for org-agent Communication Protocol (OACP)")
-(in-suite oacp-suite)
+(def-suite harness-protocol-suite
+  :description "Test suite for org-agent Communication Protocol (Harness Protocol)")
+(in-suite harness-protocol-suite)
 
 (test test-framing
   "Verify that messages are correctly prefixed with a 6-character hex length."