feat(config): add validation.on-create and validation.on-sync config options (bd-t7jq)

Add .beads/config.yaml support for template validation settings:
- validation.on-create: warn|error|none (default: none)
- validation.on-sync: warn|error|none (default: none)

When set to "warn", issues missing required sections (based on type) show
warnings but operations proceed. When set to "error", operations fail.

Implementation:
- Add validation keys to YamlOnlyKeys in yaml_config.go
- Add defaults in config.go
- Wire up bd create to check validation.on-create config
- Wire up bd sync to run validation before export
- Add tests for config loading
- Update CONFIG.md documentation

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

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

cmd/bd/sync.go

@@ -287,6 +287,11 @@ Use --merge to merge the sync branch back to main branch.`,
 					}
 				}
 
+				// Template validation before export (bd-t7jq)
+				if err := validateOpenIssuesForSync(ctx); err != nil {
+					FatalError("%v", err)
+				}
+
 				fmt.Println("→ Exporting pending changes to JSONL...")
 				if err := exportToJSONL(ctx, jsonlPath); err != nil {
 					FatalError("exporting: %v", err)