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/daemon/daemon.go

@@ -417,7 +417,8 @@ func (d *Daemon) ensureWitnessesRunning() {
 
 // ensureWitnessRunning ensures the witness for a specific rig is running.
 func (d *Daemon) ensureWitnessRunning(rigName string) {
-	agentID := beads.WitnessBeadID(rigName)
+	prefix := config.GetRigPrefix(d.config.TownRoot, rigName)
+	agentID := beads.WitnessBeadIDWithPrefix(prefix, rigName)
 	sessionName := "gt-" + rigName + "-witness"
 
 	// Check agent bead state (ZFC: trust what agent reports)
@@ -503,7 +504,8 @@ func (d *Daemon) ensureRefineriesRunning() {
 
 // ensureRefineryRunning ensures the refinery for a specific rig is running.
 func (d *Daemon) ensureRefineryRunning(rigName string) {
-	agentID := beads.RefineryBeadID(rigName)
+	prefix := config.GetRigPrefix(d.config.TownRoot, rigName)
+	agentID := beads.RefineryBeadIDWithPrefix(prefix, rigName)
 	sessionName := "gt-" + rigName + "-refinery"
 
 	// Check agent bead state (ZFC: trust what agent reports)