docs: remove 3 stale registration references from README

The /register endpoint was removed in this PR but three references
to it survived in README.md:
- 'session/register request' → 'session request'
- 'session creation, registration, or login' → 'session creation or login'
- 'session creation, registration, or NICK change' → 'session creation or NICK change'

README.md

@@ -212,13 +212,13 @@ Each session has an IRC-style hostmask composed of three parts:
 
 - **nick** — the user's current nick (changes with `NICK` command)
 - **username** — an ident-like identifier set at session creation (optional
-  `username` field in the session/register request; defaults to the nick)
+  `username` field in the session request; defaults to the nick)
 - **hostname** — automatically resolved via reverse DNS of the connecting
   client's IP address at session creation time
 - **ip** — the real IP address of the session creator, extracted from
   `X-Forwarded-For`, `X-Real-IP`, or `RemoteAddr`
 
-Each **client connection** (created at session creation, registration, or login)
+Each **client connection** (created at session creation or login)
 also stores its own **ip** and **hostname**, allowing the server to track the
 network origin of each individual client independently from the session.
 Client-level IP and hostname are **not displayed to regular users**. They are
@@ -2468,8 +2468,7 @@ Clients should handle these message commands from the queue:
 - **HTTP 401**: Auth cookie expired or invalid. Re-create session or
   re-login (if a password was set).
 - **HTTP 404**: Channel or user not found.
-- **HTTP 409**: Nick already taken (on session creation, registration, or
-  NICK change).
+- **HTTP 409**: Nick already taken (on session creation or NICK change).
 - **HTTP 400**: Malformed request. Check the `error` field in the response.
 - **Network errors**: Back off exponentially (1s, 2s, 4s, ..., max 30s).