feat(guilds): a third place on the guild page, and where that page lives

Two lines only core cannot supply for itself.

`uo.guild.header` is a third declared slot, at the top of the page, for core's
per-Team notification control. A third rather than a corner of the feed because a
slot holds one component and the first fill wins: the control is an action ON this
page and the other two are content IN it, and separate slots are what let this
module say so.

`pageUrlTemplate` tells core where a guild page actually is. Teams are a contract
primitive with no core surface — core owns the tables and the access rules, this
module owns the word "guild" and therefore the page — which leaves core unable to
write a link to one. A notification email that cannot take you to the thread it is
about is most of the way to useless. Core substitutes `{externalId}` and does
nothing else with it; a template naming its own host is refused at registration.

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

server/model/teamProvider/teamProvider.model.js

@@ -317,4 +317,18 @@ async function projectRoster(externalId, members, viewer) {
   }
 }
 
-module.exports = { getTeams, getTeamMembers, getTeamLeaders, projectRoster, boardIsCurrent }
+// Where core should point a link at a guild (MODULE_API 1.6.0, TEAMS.md §6.4).
+//
+// **Core cannot work this out for itself, and it is not supposed to.** Teams are
+// a contract primitive with no core surface — this module owns the guild page,
+// because core does not own the word "guild" — so the one thing core needs back
+// is where the page it does not own actually lives. A notification email that
+// cannot link to the thread it is about is most of the way to useless.
+//
+// A relative path with `{externalId}` substituted, matching `Guild.jsx`'s route
+// (`/uo/guilds/:id`). Core does the substitution and nothing else with it; a
+// template naming its own host is refused at registration, which is why this is
+// data and not a callback.
+const pageUrlTemplate = '/uo/guilds/{externalId}'
+
+module.exports = { getTeams, getTeamMembers, getTeamLeaders, projectRoster, boardIsCurrent, pageUrlTemplate }