feat(mail): Add isAnnounceAddress() and parseAnnounceName() helpers

Add announce address detection to internal/mail/router.go following
the same pattern as isListAddress/parseListName and isQueueAddress/
parseQueueName.

Added:
- isAnnounceAddress(address string) bool - returns true for 'announce:' prefix
- parseAnnounceName(address string) string - extracts channel name
- ErrUnknownAnnounce error variable

(gt-pn2fq)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mail/router.go

@@ -20,6 +20,9 @@ var ErrUnknownList = errors.New("unknown mailing list")
 // ErrUnknownQueue indicates a queue name was not found in configuration.
 var ErrUnknownQueue = errors.New("unknown queue")
 
+// ErrUnknownAnnounce indicates an announce channel name was not found in configuration.
+var ErrUnknownAnnounce = errors.New("unknown announce channel")
+
 // Router handles message delivery via beads.
 // It routes messages to the correct beads database based on address:
 // - Town-level (mayor/, deacon/) -> {townRoot}/.beads
@@ -73,6 +76,16 @@ func parseQueueName(address string) string {
 	return strings.TrimPrefix(address, "queue:")
 }
 
+// isAnnounceAddress returns true if the address uses announce:name syntax.
+func isAnnounceAddress(address string) bool {
+	return strings.HasPrefix(address, "announce:")
+}
+
+// parseAnnounceName extracts the announce channel name from an announce:name address.
+func parseAnnounceName(address string) string {
+	return strings.TrimPrefix(address, "announce:")
+}
+
 // expandList returns the recipients for a mailing list.
 // Returns ErrUnknownList if the list is not found.
 func (r *Router) expandList(listName string) ([]string, error) {