feat(dates): add --due and --defer timestamp options with natural language parsing (#847)

* feat(dates): add due date schema and --due flag

- Add due_at and defer_until columns to issues table via migration 035
- Implement --due flag on create command with ISO date parsing
- Extend RPC protocol and daemon to pass DueAt from CLI to storage
- Display DueAt and DeferUntil in show command output
- Update Issue type with new date fields

Users can now set due dates when creating issues, enabling deadline-based
task management.

* feat(dates): add compact duration parser (+6h, +1d, +2w)

- Create internal/timeparsing package with layered parser architecture
- Implement parseCompactDuration with regex pattern [+-]?\d+[hdwmy]
- Add comprehensive test suite (22 cases) for duration parsing
- Integrate into create.go with fallback to ISO format

Supports hours (h), days (d), weeks (w), months (m), and years (y).
Negative values allowed for past dates.

* feat(dates): add NLP parsing for natural language dates

Integrate olebedev/when library for natural language time expressions.
The layered parser now handles: compact duration → absolute formats → NLP.

Changes:
- Add olebedev/when dependency for NLP parsing
- Implement ParseNaturalLanguage and ParseRelativeTime functions
- Reorder layers: absolute formats before NLP to avoid misinterpretation
- Simplify create.go to use unified ParseRelativeTime
- Add comprehensive NLP test coverage (22 test cases)

Supports: tomorrow, next monday, in 3 days, 3 days ago

* feat(dates): add --defer flag to create/update/defer commands

Add time-based deferral support alongside existing status-based defer.
Issues can now be hidden from bd ready until a specific time.

Changes:
- Add --defer flag to bd create (sets defer_until on creation)
- Add --due and --defer flags to bd update (modify existing issues)
- Add --until flag to bd defer (combines status=deferred with defer_until)
- Add DueAt/DeferUntil fields to UpdateArgs in protocol.go

Supports: +1h, tomorrow, next monday, 2025-01-15

* feat(dates): add defer_until filtering to ready command

Add time-based deferral support to bd ready:

- Add --include-deferred flag to show issues with future defer_until
- Filter out issues where defer_until > now by default
- Update undefer to clear defer_until alongside status change
- Add IncludeDeferred to WorkFilter and RPC ReadyArgs

Part of GH#820: Relative Date Parsing (Phase 5)

* feat(dates): add polish and tests for relative date parsing

Add user-facing warnings when defer date is in the past to help catch
common mistakes. Expand help text with format examples and document
the olebedev/when September parsing quirk.

Tests:
- TestCreateSuite/WithDueAt, WithDeferUntil, WithBothDueAndDefer
- TestReadyWorkDeferUntil (ExcludesFutureDeferredByDefault, IncludeDeferredShowsAll)

Docs:
- CLAUDE.md quick reference updated with new flags
- Help text examples for --due, --defer on create/update

Closes: Phase 6 of beads-820-relative-dates spec

* feat(list): add time-based query filters for defer/due dates

Add --deferred, --defer-before, --defer-after, --due-before, --due-after,
and --overdue flags to bd list command. All date filters now support
relative time expressions (+6h, tomorrow, next monday) via the
timeparsing package.

Filters:
- --deferred: issues with defer_until set
- --defer-before/after: filter by defer_until date range
- --due-before/after: filter by due_at date range
- --overdue: due_at in past AND status != closed

Existing date filters (--created-after, etc.) now also support relative
time expressions through updated parseTimeFlag().

* build(nix): update vendorHash for olebedev/when dependency

The olebedev/when library was added for natural language date parsing
(GH#820). This changes go.sum, requiring an updated vendorHash in the
Nix flake configuration.

cmd/bd/create_test.go

@@ -486,6 +486,103 @@ func TestCreateSuite(t *testing.T) {
 		}
 	})
 
+	// GH#820: Tests for DueAt and DeferUntil fields
+	t.Run("WithDueAt", func(t *testing.T) {
+		// Create issue with due date
+		dueTime := time.Now().Add(24 * time.Hour) // Due in 24 hours
+		issue := &types.Issue{
+			Title:     "Issue with due date",
+			Priority:  1,
+			Status:    types.StatusOpen,
+			IssueType: types.TypeTask,
+			DueAt:     &dueTime,
+			CreatedAt: time.Now(),
+		}
+
+		if err := s.CreateIssue(ctx, issue, "test"); err != nil {
+			t.Fatalf("failed to create issue with due date: %v", err)
+		}
+
+		// Retrieve and verify
+		retrieved, err := s.GetIssue(ctx, issue.ID)
+		if err != nil {
+			t.Fatalf("failed to get issue: %v", err)
+		}
+
+		if retrieved.DueAt == nil {
+			t.Fatal("expected DueAt to be set")
+		}
+		// Compare with 1-second tolerance for database round-trip
+		diff := retrieved.DueAt.Sub(dueTime)
+		if diff < -time.Second || diff > time.Second {
+			t.Errorf("DueAt mismatch: got %v, want %v", retrieved.DueAt, dueTime)
+		}
+	})
+
+	t.Run("WithDeferUntil", func(t *testing.T) {
+		// Create issue with defer_until
+		deferTime := time.Now().Add(2 * time.Hour) // Defer for 2 hours
+		issue := &types.Issue{
+			Title:      "Issue with defer",
+			Priority:   1,
+			Status:     types.StatusOpen,
+			IssueType:  types.TypeTask,
+			DeferUntil: &deferTime,
+			CreatedAt:  time.Now(),
+		}
+
+		if err := s.CreateIssue(ctx, issue, "test"); err != nil {
+			t.Fatalf("failed to create issue with defer: %v", err)
+		}
+
+		// Retrieve and verify
+		retrieved, err := s.GetIssue(ctx, issue.ID)
+		if err != nil {
+			t.Fatalf("failed to get issue: %v", err)
+		}
+
+		if retrieved.DeferUntil == nil {
+			t.Fatal("expected DeferUntil to be set")
+		}
+		// Compare with 1-second tolerance for database round-trip
+		diff := retrieved.DeferUntil.Sub(deferTime)
+		if diff < -time.Second || diff > time.Second {
+			t.Errorf("DeferUntil mismatch: got %v, want %v", retrieved.DeferUntil, deferTime)
+		}
+	})
+
+	t.Run("WithBothDueAndDefer", func(t *testing.T) {
+		// Create issue with both due and defer
+		dueTime := time.Now().Add(48 * time.Hour)  // Due in 48 hours
+		deferTime := time.Now().Add(24 * time.Hour) // Defer for 24 hours
+		issue := &types.Issue{
+			Title:      "Issue with both due and defer",
+			Priority:   1,
+			Status:     types.StatusOpen,
+			IssueType:  types.TypeTask,
+			DueAt:      &dueTime,
+			DeferUntil: &deferTime,
+			CreatedAt:  time.Now(),
+		}
+
+		if err := s.CreateIssue(ctx, issue, "test"); err != nil {
+			t.Fatalf("failed to create issue: %v", err)
+		}
+
+		// Retrieve and verify both fields
+		retrieved, err := s.GetIssue(ctx, issue.ID)
+		if err != nil {
+			t.Fatalf("failed to get issue: %v", err)
+		}
+
+		if retrieved.DueAt == nil {
+			t.Fatal("expected DueAt to be set")
+		}
+		if retrieved.DeferUntil == nil {
+			t.Fatal("expected DeferUntil to be set")
+		}
+	})
+
 	t.Run("DiscoveredFromInheritsSourceRepo", func(t *testing.T) {
 		// Create a parent issue with a custom source_repo
 		parent := &types.Issue{