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/beads/fields.go

@@ -509,7 +509,7 @@ func FormatSynthesisFields(fields *SynthesisFields) string {
 type RoleConfig struct {
 	// SessionPattern defines how to derive tmux session name.
 	// Supports placeholders: {rig}, {name}, {role}
-	// Examples: "gt-mayor", "gt-{rig}-{role}", "gt-{rig}-{name}"
+	// Examples: "hq-mayor", "hq-deacon", "gt-{rig}-{role}", "gt-{rig}-{name}"
 	SessionPattern string
 
 	// WorkDirPattern defines the working directory relative to town root.