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>

integrations/beads-mcp/src/beads_mcp/server.py

@@ -372,7 +372,7 @@ async def get_tool_info(tool_name: str) -> dict[str, Any]:
             "name": "list",
             "description": "List all issues with optional filters",
             "parameters": {
-                "status": "open|in_progress|blocked|closed (optional)",
+                "status": "open|in_progress|blocked|deferred|closed (optional)",
                 "priority": "int 0-4 (optional)",
                 "issue_type": "bug|feature|task|epic|chore (optional)",
                 "assignee": "str (optional)",
@@ -413,7 +413,7 @@ async def get_tool_info(tool_name: str) -> dict[str, Any]:
             "description": "Update an existing issue",
             "parameters": {
                 "issue_id": "str (required)",
-                "status": "open|in_progress|blocked|closed (optional)",
+                "status": "open|in_progress|blocked|deferred|closed (optional)",
                 "priority": "int 0-4 (optional)",
                 "assignee": "str (optional)",
                 "title": "str (optional)",