Rename ephemeral -> wisp terminology throughout Gas Town

- .beads-ephemeral/ -> .beads-wisp/
- Rename doctor checks: EphemeralCheck -> WispCheck
- Update all docs to use 'transient' for polecats, 'wisp' for molecules
- Preserve 'ephemeral' only as descriptive adjective for wisps
- Steam engine metaphor: wisps are steam vapors that dissipate

Part of Christmas launch wisp terminology unification.

internal/daemon/daemon.go

@@ -165,9 +165,9 @@ var deaconMOTDMessages = []string{
 	"Thanks for keeping the town running!",
 	"You are Gas Town's most critical role.",
 	"You are the heart of Gas Town! Be watchful!",
-	"Tip: Polecats are ephemeral - spawn freely, kill liberally.",
+	"Tip: Polecats are transient - spawn freely, kill liberally.",
 	"Tip: Witnesses monitor polecats; you monitor witnesses.",
-	"Tip: Wisps are ephemeral molecules for patrol cycles.",
+	"Tip: Wisps are transient molecules for patrol cycles.",
 	"The town sleeps soundly because you never do.",
 	"Tip: Mayor handles cross-rig coordination; you handle health.",
 	"Your vigilance keeps the agents honest.",