refactor: rename Ephemeral → Wisp (Steam Engine metaphor)

Wisp = ephemeral vapor produced by the Steam Engine (Gas Town).
This aligns with the metaphor:
- Claude = Fire
- Claude Code = Steam
- Gas Town = Steam Engine
- Wisps = ephemeral vapor it produces

Changes:
- types.Issue.Ephemeral → types.Issue.Wisp
- types.IssueFilter.Ephemeral → types.IssueFilter.Wisp
- JSON field: "ephemeral" → "wisp"
- CLI flag: --ephemeral → --wisp (bd cleanup)
- All tests updated

Note: SQLite column remains "ephemeral" (no migration needed).
This is a breaking change for JSON consumers using 0.33.0.

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

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

cmd/bd/mol_run.go

@@ -89,7 +89,7 @@ func runMolRun(cmd *cobra.Command, args []string) {
 		os.Exit(1)
 	}
 
-	// Spawn the molecule with actor as assignee (ephemeral for cleanup - bd-2vh3)
+	// Spawn the molecule with actor as assignee (wisp for cleanup - bd-2vh3)
 	result, err := spawnMolecule(ctx, store, subgraph, vars, actor, actor, true)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "Error spawning molecule: %v\n", err)