feat(status): add deferred status for icebox issues (bd-4jr)

Add 'deferred' as a valid issue status for issues that are deliberately
put on ice - not blocked by dependencies, just postponed for later.

Changes:
- Add StatusDeferred constant and update IsValid() validation
- Add DeferredIssues to Statistics struct with counting in both SQLite
  and memory storage
- Add 'bd defer' command to set status to deferred
- Add 'bd undefer' command to restore status to open
- Update help text across list, search, count, dep, stale, and config
- Update MCP server models and tools to accept deferred status
- Add deferred to blocker status checks (schema, cache, ready, compact)
- Add StatusDeferred to public API exports (beads.go, internal/beads)
- Add snowflake styling for deferred in dep tree and graph views

Semantics:
- deferred vs blocked: deferred is a choice, blocked is forced
- deferred vs closed: deferred will be revisited, closed is done
- Deferred issues excluded from 'bd ready' (already works since
  default filter only includes open/in_progress)
- Deferred issues still block dependents (they are not done!)
- Deferred issues visible in 'bd list' and 'bd stale'

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

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

cmd/bd/dep.go

@@ -497,6 +497,8 @@ func getStatusEmoji(status types.Status) string {
 		return "◧" // U+25E7 Square Left Half Black
 	case types.StatusBlocked:
 		return "⚠" // U+26A0 Warning Sign
+	case types.StatusDeferred:
+		return "❄" // U+2744 Snowflake (on ice)
 	case types.StatusClosed:
 		return "☑" // U+2611 Ballot Box with Check
 	default:
@@ -746,7 +748,7 @@ func init() {
 	depTreeCmd.Flags().IntP("max-depth", "d", 50, "Maximum tree depth to display (safety limit)")
 	depTreeCmd.Flags().Bool("reverse", false, "Show dependent tree (deprecated: use --direction=up)")
 	depTreeCmd.Flags().String("direction", "", "Tree direction: 'down' (dependencies), 'up' (dependents), or 'both'")
-	depTreeCmd.Flags().String("status", "", "Filter to only show issues with this status (open, in_progress, blocked, closed)")
+	depTreeCmd.Flags().String("status", "", "Filter to only show issues with this status (open, in_progress, blocked, deferred, closed)")
 	depTreeCmd.Flags().String("format", "", "Output format: 'mermaid' for Mermaid.js flowchart")
 	// Note: --json flag is defined as a persistent flag in main.go, not here