Refactor: extract duplicated validation and flag logic (bd-g5p7)

- Created internal/validation package for centralized validation logic
- Created cmd/bd/flags.go for shared flag registration
- Updated create and update commands to use shared logic
- Added support for 'P1' style priority to update command
- Added tests for validation logic

Amp-Thread-ID: https://ampcode.com/threads/T-c8d369a3-32f0-42a0-96d1-fd589e89bd6b
Co-authored-by: Amp <amp@ampcode.com>

internal/validation/bead.go

@@ -0,0 +1,47 @@
+package validation
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/steveyegge/beads/internal/types"
+)
+
+// ParsePriority extracts and validates a priority value from content.
+// Supports both numeric (0-4) and P-prefix format (P0-P4).
+// Returns the parsed priority (0-4) or -1 if invalid.
+func ParsePriority(content string) int {
+	content = strings.TrimSpace(content)
+	
+	// Handle "P1", "P0", etc. format
+	if strings.HasPrefix(strings.ToUpper(content), "P") {
+		content = content[1:] // Strip the "P" prefix
+	}
+	
+	var p int
+	if _, err := fmt.Sscanf(content, "%d", &p); err == nil && p >= 0 && p <= 4 {
+		return p
+	}
+	return -1 // Invalid
+}
+
+// ParseIssueType extracts and validates an issue type from content.
+// Returns the validated type or error if invalid.
+func ParseIssueType(content string) (types.IssueType, error) {
+	issueType := types.IssueType(strings.TrimSpace(content))
+
+	// Validate issue type
+	validTypes := map[types.IssueType]bool{
+		types.TypeBug:     true,
+		types.TypeFeature: true,
+		types.TypeTask:    true,
+		types.TypeEpic:    true,
+		types.TypeChore:   true,
+	}
+
+	if !validTypes[issueType] {
+		return types.TypeTask, fmt.Errorf("invalid issue type: %s", content)
+	}
+
+	return issueType, nil
+}