refactor: add TownBeadsPrefix constant for hq- prefix (gt-3jnnu)

Add TownBeadsPrefix constant to agent_ids.go to centralize the "hq"
prefix string used for town-level agent beads. This makes prefix
changes easier and reduces string duplication.

Also update agent_beads_check.go to use the helper functions instead
of hardcoded strings for consistency.

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

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

internal/doctor/agent_beads_check.go

@@ -169,7 +169,7 @@ func (c *AgentBeadsCheck) Fix(ctx *CheckContext) error {
 			RoleType:   "deacon",
 			Rig:        "",
 			AgentState: "idle",
-			RoleBead:   "hq-deacon-role",
+			RoleBead:   beads.DeaconRoleBeadIDTown(),
 		}
 		desc := "Deacon (daemon beacon) - receives mechanical heartbeats, runs town plugins and monitoring."
 		if _, err := townBd.CreateAgentBead(deaconID, desc, fields); err != nil {
@@ -183,7 +183,7 @@ func (c *AgentBeadsCheck) Fix(ctx *CheckContext) error {
 			RoleType:   "mayor",
 			Rig:        "",
 			AgentState: "idle",
-			RoleBead:   "hq-mayor-role",
+			RoleBead:   beads.MayorRoleBeadIDTown(),
 		}
 		desc := "Mayor - global coordinator, handles cross-rig communication and escalations."
 		if _, err := townBd.CreateAgentBead(mayorID, desc, fields); err != nil {