feat(validation): add ValidateTemplate() function (bd-615z)

Implements template validation for bd lint and bd create --validate:
- ValidateTemplate() checks description for required sections
- LintIssue() validates an existing issue
- TemplateError provides actionable missing section details
- Case-insensitive, flexible heading matching

Also fixes: close.go missing session parameter in CloseIssue calls

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/validation/template.go

@@ -0,0 +1,82 @@
+package validation
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/steveyegge/beads/internal/types"
+)
+
+// MissingSection describes a section that should be present but isn't.
+type MissingSection struct {
+	Heading string // The expected heading, e.g., "## Steps to Reproduce"
+	Hint    string // Guidance for what to include
+}
+
+// TemplateError is returned when template validation fails.
+// It contains all missing sections for a single error report.
+type TemplateError struct {
+	IssueType types.IssueType
+	Missing   []MissingSection
+}
+
+func (e *TemplateError) Error() string {
+	if len(e.Missing) == 0 {
+		return ""
+	}
+	var b strings.Builder
+	fmt.Fprintf(&b, "missing required sections for %s:", e.IssueType)
+	for _, m := range e.Missing {
+		fmt.Fprintf(&b, "\n  - %s (%s)", m.Heading, m.Hint)
+	}
+	return b.String()
+}
+
+// ValidateTemplate checks if the description contains all required sections
+// for the given issue type. Returns nil if validation passes or if the
+// issue type has no required sections.
+//
+// Section matching is case-insensitive and looks for the heading text
+// anywhere in the description (doesn't require exact markdown format).
+func ValidateTemplate(issueType types.IssueType, description string) error {
+	required := issueType.RequiredSections()
+	if len(required) == 0 {
+		return nil
+	}
+
+	descLower := strings.ToLower(description)
+	var missing []MissingSection
+
+	for _, section := range required {
+		// Extract the heading text without markdown prefix for flexible matching
+		// e.g., "## Steps to Reproduce" -> "steps to reproduce"
+		headingText := strings.TrimPrefix(section.Heading, "## ")
+		headingText = strings.TrimPrefix(headingText, "# ")
+		headingLower := strings.ToLower(headingText)
+
+		if !strings.Contains(descLower, headingLower) {
+			missing = append(missing, MissingSection{
+				Heading: section.Heading,
+				Hint:    section.Hint,
+			})
+		}
+	}
+
+	if len(missing) > 0 {
+		return &TemplateError{
+			IssueType: issueType,
+			Missing:   missing,
+		}
+	}
+	return nil
+}
+
+// LintIssue checks an existing issue for missing template sections.
+// Unlike ValidateTemplate, this operates on a full Issue struct.
+// Returns nil if the issue passes validation or has no requirements.
+func LintIssue(issue *types.Issue) error {
+	if issue == nil {
+		return nil
+	}
+	return ValidateTemplate(issue.IssueType, issue.Description)
+}