Refine domain classes: behavior tags, extra-tag passthrough, cleanups

Iterate on @welshman/domain following review:

- base: add `group`/`protect`/`expires` behavior tags (parsed in base, emitted
  via addBehaviorTags before hashing) and an `extraTags` passthrough (opt-in via
  reservedTagKeys) so tag carry-over lives in one place; migrate Handler, Comment,
  Thread onto it. Comment gains nested root/parent ref structs + setters.
- List: fix inverted keepTags; add clearTags/clearPublicTags/clearPrivateTags and
  use them in the relay/server list setters.
- RelayList: preserve complementary read/write capability instead of dropping
  modeless entries.
- Split Relay/Room membership ops into per-kind classes (RelayAddMember/
  RelayRemoveMember, RoomAddMember/RoomRemoveMember) over a shared base.
- TimeEvent (renamed from CalendarEvent): derive "D" day tags in toTemplate.
- Feed: default to an empty feed, fail parse when the "feed" tag is missing.
- RelaySet added; CommunityList renamed to GroupList; predicate bare add/remove
  mutators; RoomMeta uses randomId; PollResponse.selections drops pollType.
- Remove ChannelList, FileServerList, Settings, and event-asserting getAddress/
  display accessors.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V67tPYdvh1qCkjEBhJGZUR

packages/domain/src/BlossomServerList.ts

@@ -25,7 +25,7 @@ export class BlossomServerList extends EncryptableList {
   }
 
   setServers(urls: string[]) {
-    this.keepTagsWithKey("server")
+    this.clearTags()
 
     return this.addPublicTags(...urls.map(url => ["server", normalizeRelayUrl(url)]))
   }