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_bond.go

@@ -283,7 +283,7 @@ func bondProtoMol(ctx context.Context, s storage.Storage, proto, mol *types.Issu
 		return nil, fmt.Errorf("missing required variables: %s (use --var)", strings.Join(missingVars, ", "))
 	}
 
-	// Spawn the proto (ephemeral by default for molecule execution - bd-2vh3)
+	// Spawn the proto (wisp by default for molecule execution - bd-2vh3)
 	spawnResult, err := spawnMolecule(ctx, s, subgraph, vars, "", actorName, true)
 	if err != nil {
 		return nil, fmt.Errorf("spawning proto: %w", err)