fix: Create and lookup rig agent beads with correct prefix

Per docs/architecture.md, Witness and Refinery are rig-level agents that
should use the rig's configured prefix (e.g., pi- for pixelforge) instead
of hardcoded "gt-".

This extends PR #183's creation fix to also fix all lookup paths:
- internal/rig/manager.go: Create agent beads in rig beads with rig prefix
- internal/daemon/daemon.go: Use rig prefix when looking up agent state
- internal/daemon/lifecycle.go: Use rig prefix for identity-to-bead mapping
- internal/cmd/sling.go: Pass townRoot for prefix lookup
- internal/cmd/unsling.go: Pass townRoot for prefix lookup
- internal/cmd/molecule_status.go: Use rig prefix for agent bead lookups
- internal/cmd/molecule_attach.go: Use rig prefix for agent bead lookups
- internal/config/loader.go: Add GetRigPrefix helper

Without this fix, the daemon would:
- Create pi-gastown-witness but look for gt-gastown-witness
- Report agents as missing/dead when they are running
- Fail to manage agent lifecycle correctly

Based on work by Johann Taberlet in PR #183.

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

Co-Authored-By: Johann Taberlet <johann.taberlet@gmail.com>
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/molecule_attach.go

@@ -87,7 +87,7 @@ func detectAgentBeadID() (string, error) {
 		return "", fmt.Errorf("cannot determine agent identity (role: %s)", roleCtx.Role)
 	}
 
-	beadID := buildAgentBeadID(identity, roleCtx.Role)
+	beadID := buildAgentBeadID(identity, roleCtx.Role, townRoot)
 	if beadID == "" {
 		return "", fmt.Errorf("cannot build agent bead ID for identity: %s", identity)
 	}