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/daemon/lifecycle_test.go

@@ -184,10 +184,10 @@ func TestIdentityToSession_Mayor(t *testing.T) {
 	d, cleanup := testDaemonWithTown(t, "ai")
 	defer cleanup()
 
-	// Mayor session name is now fixed (one per machine, no town qualifier)
+	// Mayor session name is now fixed (one per machine, uses hq- prefix)
 	result := d.identityToSession("mayor")
-	if result != "gt-mayor" {
-		t.Errorf("identityToSession('mayor') = %q, expected 'gt-mayor'", result)
+	if result != "hq-mayor" {
+		t.Errorf("identityToSession('mayor') = %q, expected 'hq-mayor'", result)
 	}
 }