Handle server-prefixed AUTHENTICATE in SASL dispatcher

[silverbucket]

Fixes #19.

Summary

• startupHandler matched AUTHENTICATE only on parts[0], so server-prefixed lines (e.g. Ergo's :ergo.test AUTHENTICATE +) were ignored and the SASL handshake stalled until the caller timed out.
• Mirrors the existing CAP / NOTICE pattern: match on parts[0] or numeric (= parts[1]), and read the payload from parts[2] when the line is prefixed.
• No other branches change — CAP, NOTICE, and numerics already handle prefixes correctly.

Test plan

• Existing mocha suite still passes (47 prior tests).
• New test: `SASL PLAIN completes when server prefixes AUTHENTICATE +`.
• New test: `SASL OAUTHBEARER acks prefixed failure challenge with AQ==`.
• Downstream: rerun Sockethub's Ergo integration harness (feat(platform-irc): add SASL OAUTHBEARER token-based authentication sockethub/sockethub#1055) against a bumped `irc-socket-sasl` to confirm PLAIN + OAUTHBEARER handshakes complete end-to-end.