[2026 Templates] Validate templates against the registry schema before use

[Nanle-code]

Objective

Fail early with field-level errors for malformed local or marketplace templates.

Target area

templates/registry.schema.json and loaders

Acceptance criteria

• The objective is implemented with clear handling for invalid input, unsupported environments, and failure paths.
• Automated tests cover the primary flow, at least one boundary case, and at least one failure case.
• User-facing documentation or developer guidance is updated, including compatibility, security, or migration notes where applicable.

Source: .github/roadmap-issues-2026-07-29.json.