Add lightweight decision records for CodeGuard

[thomas-bartlett]

Summary

Add a lightweight decision-record process so contributors and AI tools can understand important CodeGuard design choices.

Scope

• Choose a repo location for decision records.
• Add a concise Markdown template.
• Include status, context, decision, consequences, and related issues or PRs.
• Add one example record or list the first candidate records.

Acceptance Criteria

• Decision-record location and naming convention are documented.
• A concise template exists.
• At least one example or candidate first record is included.
• The process is lightweight enough for maintainers to use.