feat: bd list defaults to non-closed issues with 50 limit (GH#788)

Changes:
- Default to excluding closed issues (use --all to include)
- Default limit of 50 issues (use --limit 0 for unlimited)
- --all flag now overrides the closed filter

This addresses agent context blowout from seeing hundreds of closed issues.

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

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

internal/types/types.go

@@ -720,6 +720,9 @@ type IssueFilter struct {
 
 	// Molecule type filtering
 	MolType *MolType // Filter by molecule type (nil = any, swarm/patrol/work)
+
+	// Status exclusion (for default non-closed behavior)
+	ExcludeStatus []Status // Exclude issues with these statuses
 }
 
 // SortPolicy determines how ready work is ordered