docs: add literate prose to naming-drift aliases and HITL gateway changes

Adds prose sections before every code block to satisfy the
prose-before-code discipline. Each backward-compatibility alias
(process-signal, perceive-gate, reason-gate, act-gate, inject-stimulus)
now has its own subsection explaining why it exists and what new code
should use instead.

Also:
- Fixes double #+end_src in core-loop-perceive.org
- Renames inject-stimulus → stimulus-inject in heartbeat-start and
  client-handle-connection (both already had aliases)
- Adds HITL interception prose to gateway-manager.org telegram/signal
  sections
- Splits Pre-Reason Handler Registry into two code blocks (defvar + defun)
  for one-per-block compliance

lisp/core-loop-reason.lisp

@@ -121,10 +121,6 @@ modified action (for approval-required or pass)."
                              :action approval-action))
         current-action)))
 
-;; Alias: reason-gate → loop-gate-reason
-(defun reason-gate (signal)
-  (loop-gate-reason signal))
-
 (defun loop-gate-reason (signal)
   (let* ((type (proto-get signal :type))
          (payload (proto-get signal :payload))
@@ -162,6 +158,9 @@ modified action (for approval-required or pass)."
                 (setf (getf signal :status) :reasoned)
                 (return signal))))))))
 
+(defun reason-gate (signal)
+  (loop-gate-reason signal))
+
 (eval-when (:compile-toplevel :load-toplevel :execute)
   (ql:quickload :fiveam :silent t))