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>

internal/types/types.go

@@ -219,6 +219,7 @@ const (
 	StatusOpen       Status = "open"
 	StatusInProgress Status = "in_progress"
 	StatusBlocked    Status = "blocked"
+	StatusDeferred   Status = "deferred" // Deliberately put on ice for later (bd-4jr)
 	StatusClosed     Status = "closed"
 	StatusTombstone  Status = "tombstone" // Soft-deleted issue (bd-vw8)
 	StatusPinned     Status = "pinned"    // Persistent bead that stays open indefinitely (bd-6v2)
@@ -227,7 +228,7 @@ const (
 // IsValid checks if the status value is valid (built-in statuses only)
 func (s Status) IsValid() bool {
 	switch s {
-	case StatusOpen, StatusInProgress, StatusBlocked, StatusClosed, StatusTombstone, StatusPinned:
+	case StatusOpen, StatusInProgress, StatusBlocked, StatusDeferred, StatusClosed, StatusTombstone, StatusPinned:
 		return true
 	}
 	return false
@@ -425,6 +426,7 @@ type Statistics struct {
 	InProgressIssues         int     `json:"in_progress_issues"`
 	ClosedIssues             int     `json:"closed_issues"`
 	BlockedIssues            int     `json:"blocked_issues"`
+	DeferredIssues           int     `json:"deferred_issues"`  // Issues on ice (bd-4jr)
 	ReadyIssues              int     `json:"ready_issues"`
 	TombstoneIssues          int     `json:"tombstone_issues"` // Soft-deleted issues (bd-nyt)
 	PinnedIssues             int     `json:"pinned_issues"`    // Persistent issues (bd-6v2)