fix(rig): check witness/state.json instead of witness/rig (gm-2ej)

Witnesses don't have git clones like other agents. The detection was
checking for witness/rig which doesn't exist. Now correctly checks for
witness/state.json which is created by AddRig.

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

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

internal/rig/types.go

@@ -37,11 +37,12 @@ type Rig struct {
 }
 
 // AgentDirs are the standard agent directories in a rig.
+// Note: witness doesn't have a /rig subdirectory (no clone needed).
 var AgentDirs = []string{
 	"polecats",
 	"crew",
 	"refinery/rig",
-	"witness/rig",
+	"witness",
 	"mayor/rig",
 }