Declare IRCv3 bot mode (+B) when the network advertises BOT= #1

IRCv3 bot mode is how a bot tells the network it is a bot. The server advertises
BOT=<letter> in RPL_ISUPPORT, the client sets that user mode on itself, and
the server then attaches a bot tag to that user's PRIVMSG/NOTICE for every
recipient that negotiated message-tags. Other bots can then leave it alone
without anybody hand-maintaining an ignore list.

eggbot never claims it. Grepping internal/ finds no BOT=, no +B and no
reference to the mode at all.

What it would take

1. Read the advertised letter. No parser change is needed:
   ISupport.ApplyTokens (internal/ircx/isupport.go:38) already stores every
   token generically, so Get("BOT") returns it today.
2. Send MODE <nick> +<letter> once registration is done and the letter is
   known, guarded so it is sent once per connection.

The ordering is the only subtlety: 005 can arrive after 001, so hooking
onWelcome (internal/bot/bot.go:370) alone is not reliable. Attempting the
claim from both the welcome and ISUPPORT paths, with a latch so it fires once,
covers both orders.

Why now

Servers are starting to implement the other half. chonkline shipped it in
iconidentify/chonkline#36, which adds message-tags,
account-tag, the +B user mode and BOT=B in 005, and sends the bot tag
per recipient. On a network like that, a bot which does not set the mode is
indistinguishable from a human to every other well-behaved client.

Recognising the tag on inbound messages is a separate change; filed separately
since each is useful without the other.