- xmpp: gate XEP-0359 stanza-id dedup on disco urn:xmpp:sid:0. Before
honoring <stanza-id by='…'/> for archive_id dedup, check that the entity
in `by` (or its domain — that's what profanity disco's on connect)
announces the namespace; fall back to no-dedup when caps are unknown,
matching the XEP §6 disco-gate (#1).
- accounts: narrow group-name sanitizer to only the characters GKeyFile
actually forbids in group headers (`[`, `]`, `\n`, `\r`); `=` and `#` are
valid inside a header and stay (#2).
- functional tests cover both branches of the disco gate — replayed
stanza-id is flagged as duplicate when sid:0 is announced, and not
flagged when it isn't.
Refs #112
