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>

internal/storage/sqlite/issues.go

@@ -28,7 +28,7 @@ func insertIssue(ctx context.Context, conn *sql.Conn, issue *types.Issue) error
 	}
 
 	wisp := 0
-	if issue.Wisp {
+	if issue.Ephemeral {
 		wisp = 1
 	}
 	pinned := 0
@@ -94,7 +94,7 @@ func insertIssues(ctx context.Context, conn *sql.Conn, issues []*types.Issue) er
 		}
 
 		wisp := 0
-		if issue.Wisp {
+		if issue.Ephemeral {
 			wisp = 1
 		}
 		pinned := 0