feat: use hq- prefix for Mayor and Deacon session names

Town-level services (Mayor, Deacon) now use hq- prefix instead of gt-:
- hq-mayor (was gt-mayor)
- hq-deacon (was gt-deacon)

This distinguishes town-level sessions from rig-level sessions which
continue to use gt- prefix (gt-gastown-witness, gt-gastown-crew-max, etc).

Changes:
- session.MayorSessionName() returns "hq-mayor"
- session.DeaconSessionName() returns "hq-deacon"
- ParseSessionName() handles both hq- and gt- prefixes
- categorizeSession() handles both prefixes
- categorizeSessions() accepts both prefixes
- Updated all tests and documentation

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

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

internal/mail/router_test.go

@@ -91,9 +91,9 @@ func TestAddressToSessionID(t *testing.T) {
 		address string
 		want    string
 	}{
-		{"mayor", "gt-mayor"},
-		{"mayor/", "gt-mayor"},
-		{"deacon", "gt-deacon"},
+		{"mayor", "hq-mayor"},
+		{"mayor/", "hq-mayor"},
+		{"deacon", "hq-deacon"},
 		{"gastown/refinery", "gt-gastown-refinery"},
 		{"gastown/Toast", "gt-gastown-Toast"},
 		{"beads/witness", "gt-beads-witness"},