fix: use client nick for synthetic JOINs and own-nick rewriting

irssi (and other IRC clients) only open a channel window when they see
a JOIN from their own nick. The synthetic JOINs were using the network
nick (e.g. pagumowa) but the client registered as tester -- mismatch.

Three changes:
- Synthetic JOIN prefix is now client_nick!user@bouncer
- 001 welcome uses the client's registered nick
- encode_nick/encode_message accept client_nick param to rewrite own
  nicks from any network to the client's nick, so irssi recognizes
  all self-actions consistently

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_namespace.py

@@ -44,6 +44,12 @@ class TestEncodeNick:
         # Own nick on another network still shown bare
         assert encode_nick("mybot2", "libera", OWN) == "mybot2"
 
+    def test_own_nick_rewritten_to_client_nick(self):
+        assert encode_nick("mybot", "libera", OWN, client_nick="tester") == "tester"
+
+    def test_foreign_nick_unaffected_by_client_nick(self):
+        assert encode_nick("user123", "libera", OWN, client_nick="tester") == "user123/libera"
+
 
 class TestEncodePrefix:
     def test_full_prefix(self):
@@ -141,3 +147,22 @@ class TestEncodeMessage:
         )
         out = encode_message(msg, "libera", OWN)
         assert out.raw is None  # raw must not carry over
+
+    def test_own_prefix_rewritten_to_client_nick(self):
+        msg = IRCMessage(
+            command="PRIVMSG",
+            params=["#test", "hello"],
+            prefix="mybot!ident@host",
+        )
+        out = encode_message(msg, "libera", OWN, client_nick="tester")
+        assert out.prefix == "tester!ident@host"
+
+    def test_namreply_own_nick_rewritten(self):
+        msg = IRCMessage(
+            command="353",
+            params=["mybot", "=", "#test", "@op mybot"],
+        )
+        out = encode_message(msg, "libera", OWN, client_nick="tester")
+        names = out.params[3].split()
+        assert names[0] == "@op/libera"
+        assert names[1] == "tester"