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>

internal/config/config.go

@@ -114,6 +114,14 @@ func Initialize() error {
 	// Create command defaults
 	v.SetDefault("create.require-description", false)
 
+	// Validation configuration defaults (bd-t7jq)
+	// Values: "warn" | "error" | "none"
+	// - "none": no validation (default, backwards compatible)
+	// - "warn": validate and print warnings but proceed
+	// - "error": validate and fail on missing sections
+	v.SetDefault("validation.on-create", "none")
+	v.SetDefault("validation.on-sync", "none")
+
 	// Git configuration defaults (GH#600)
 	v.SetDefault("git.author", "")        // Override commit author (e.g., "beads-bot <beads@example.com>")
 	v.SetDefault("git.no-gpg-sign", false) // Disable GPG signing for beads commits