fix: improve YAML formatting and resolve gosec lint warnings

[indaco]

Description

Fix YAML output formatting when generating .sley.yaml via sley init and resolve gosec lint warnings.

YAML formatting:

Replace yaml.Marshal with yaml.MarshalWithOptions(..., yaml.Indent(2), yaml.IndentSequence(true)) so that YAML sequences (e.g. files under dependency-check) are properly indented:

  # before                     # after
  files:                       files:
  - path: package.json           - path: package.json
    field: version                 field: version

Gosec fixes:

• G122 (workspace.go): Use os.OpenRoot for symlink-safe file reads inside WalkDir callback
• G703 (updater.go, generator.go): Suppress path traversal taint warnings for internally-resolved config/changelog paths
• G118 (error_recovery_test.go, executor_test.go): Add defer cancel() for context cancellation

Related Issue

• None

Notes for Reviewers

• None