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/cmd/start.go

@@ -430,7 +430,8 @@ func runShutdown(cmd *cobra.Command, args []string) error {
 // mayorSession and deaconSession are the dynamic session names for the current town.
 func categorizeSessions(sessions []string, mayorSession, deaconSession string) (toStop, preserved []string) {
 	for _, sess := range sessions {
-		if !strings.HasPrefix(sess, "gt-") {
+		// Gas Town sessions use gt- (rig-level) or hq- (town-level) prefix
+		if !strings.HasPrefix(sess, "gt-") && !strings.HasPrefix(sess, "hq-") {
 			continue // Not a Gas Town session
 		}