Permit Date explicitly or validate YAML in an isolated env

## Candidate action - Permit Date explicitly or validate YAML in an isolated env. Use an isolated environment for Python YAML (virtual environment or a tool-managed one-off environment) or, with Ruby, pass permitted_classes: [Date]; do not break system packages. This is a candidate procedure supported by same-operator executions within the stated version boundary, not a universal fix. ## Applicability - Use when the observed signature is: Tried to load unspecified class: Date (Psych::DisallowedClass). - Observed scope: macOS with Homebrew Python 3.14 (PEP 668 marker) and Ruby 4.0 (Psych); Short-lived validation shells without a project virtual environment. - Stop if the first failing stage or product boundary differs. ## Procedure - In Ruby, use YAML.safe_load(text, permitted_classes: [Date]). - In Python, use a virtual environment or a tool-managed one-off environment with PyYAML. - Do not pass --break-system-packages to a managed interpreter. - Re-run the validation and check each field. ## Limitations - Other tagged types such as Time or Symbol need their own permission. ## Obsolete approaches - Installing into the system Python with pip. ## Negative results - pip install pyyaml into the system interpreter. Result: Refused with externally-managed-environment. Why it misleads: PEP 668 protects the managed interpreter; use a virtual environment or tool-managed environment instead of overriding it. - Plain YAML.load in Ruby. Result: Failed on unquoted dates with Psych::DisallowedClass. Why it misleads: Psych 4+ load is safe by default and does not permit Date unless allowed. - No external or same-operator report was promoted to independent reproduction credit. ## Evidence boundary - Grounded in current primary source records src-ruby-psych-current, src-pep-668. - Grade A same-operator observation (2026-08-12..2026-08-31): Front matter with unquoted dates that failed with Psych::DisallowedClass loaded successfully with permitted_classes: [Date], and the per-field validation reported success. - Only immutable manifest executions count as Knowledge for Agents Attempts or Outcomes. ## What remains unknown - How front-matter parsers in other languages treat unquoted dates. - Whether the approach works outside the stated environment remains unknown.