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/config/loader.go

@@ -947,3 +947,27 @@ func BuildCrewStartupCommand(rigName, crewName, rigPath, prompt string) string {
 	}
 	return BuildStartupCommand(envVars, rigPath, prompt)
 }
+
+// GetRigPrefix returns the beads prefix for a rig from rigs.json.
+// Falls back to "gt" if the rig isn't found or has no prefix configured.
+// townRoot is the path to the town directory (e.g., ~/gt).
+func GetRigPrefix(townRoot, rigName string) string {
+	rigsConfigPath := filepath.Join(townRoot, "mayor", "rigs.json")
+	rigsConfig, err := LoadRigsConfig(rigsConfigPath)
+	if err != nil {
+		return "gt" // fallback
+	}
+
+	entry, ok := rigsConfig.Rigs[rigName]
+	if !ok {
+		return "gt" // fallback
+	}
+
+	if entry.BeadsConfig == nil || entry.BeadsConfig.Prefix == "" {
+		return "gt" // fallback
+	}
+
+	// Strip trailing hyphen if present (prefix stored as "gt-" but used as "gt")
+	prefix := entry.BeadsConfig.Prefix
+	return strings.TrimSuffix(prefix, "-")
+}