fix(architecture): record whether the Reviewer Gate ran in spine frontmatter

[gmoncor]

What: Add a reviewed frontmatter field to spine-template.md (default false, set true once the Reviewer Gate has run) and update reviewer-gate.md to set it when the gate runs and leave/set it false when explicitly skipped.

Why: A throwaway prototype may run the Reviewer Gate quietly or skip it entirely, but the finished spine document carries no visible marker of which happened.

How:

• Add reviewed: false next to the existing status field in spine-template.md.
• Update reviewer-gate.md to set the field to true when the gate runs, at any tier.

Testing: Read-through diff; additive frontmatter field, no existing consumer of spine frontmatter breaks since the field is new and optional.

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