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/export.go

@@ -362,7 +362,7 @@ Examples:
 		// Wisps exist only in SQLite and are shared via .beads/redirect, not JSONL.
 		filtered := make([]*types.Issue, 0, len(issues))
 		for _, issue := range issues {
-			if !issue.Wisp {
+			if !issue.Ephemeral {
 				filtered = append(filtered, issue)
 			}
 		}