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/export.go

@@ -346,11 +346,11 @@ Examples:
 			}
 		}
 
-		// Filter out ephemeral issues - they should never be exported to JSONL (bd-687g)
-		// Ephemeral issues exist only in SQLite and are shared via .beads/redirect, not JSONL.
+		// Filter out wisps - they should never be exported to JSONL (bd-687g)
+		// 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.Ephemeral {
+			if !issue.Wisp {
 				filtered = append(filtered, issue)
 			}
 		}