feat: Rename 'wisp' to 'ephemeral' in beads API (bd-o18s)

BREAKING CHANGE: API field and CLI command renamed

- types.Issue.Wisp → types.Issue.Ephemeral
- JSON field: "wisp" → "ephemeral"
- CLI: bd wisp → bd ephemeral
- Flags: --wisp → --ephemeral
- ID prefix: wisp → eph

The SQLite column already uses 'ephemeral' so no schema migration needed.

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

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

cmd/bd/sync_export.go

@@ -65,7 +65,7 @@ func exportToJSONL(ctx context.Context, jsonlPath string) error {
 	// This prevents "zombie" issues that resurrect after mol squash deletes them.
 	filteredIssues := make([]*types.Issue, 0, len(issues))
 	for _, issue := range issues {
-		if issue.Wisp {
+		if issue.Ephemeral {
 			continue
 		}
 		filteredIssues = append(filteredIssues, issue)