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/storage/sqlite/compact.go

@@ -78,7 +78,7 @@ func (s *SQLiteStorage) GetTier1Candidates(ctx context.Context) ([]*CompactionCa
 		  COUNT(DISTINCT dt.dependent_id) as dependent_count
 		FROM issues i
 		LEFT JOIN dependent_tree dt ON i.id = dt.issue_id
-		  AND dt.dependent_status IN ('open', 'in_progress', 'blocked')
+		  AND dt.dependent_status IN ('open', 'in_progress', 'blocked', 'deferred')
 		  AND dt.depth <= ?
 		WHERE i.status = 'closed'
 		  AND i.closed_at IS NOT NULL
@@ -163,7 +163,7 @@ func (s *SQLiteStorage) GetTier2Candidates(ctx context.Context) ([]*CompactionCa
 		    JOIN issues dep ON d.issue_id = dep.id
 		    WHERE d.depends_on_id = i.id
 		      AND d.type = 'blocks'
-		      AND dep.status IN ('open', 'in_progress', 'blocked')
+		      AND dep.status IN ('open', 'in_progress', 'blocked', 'deferred')
 		  )
 		ORDER BY i.closed_at ASC
 	`