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/config.go

@@ -32,7 +32,7 @@ Custom Status States:
     bd config set status.custom "awaiting_review,awaiting_testing,awaiting_docs"
 
   This enables issues to use statuses like 'awaiting_review' in addition to
-  the built-in statuses (open, in_progress, blocked, closed).
+  the built-in statuses (open, in_progress, blocked, deferred, closed).
 
 Examples:
   bd config set jira.url "https://company.atlassian.net"