docs: update docs/architecture/workflows.md PR Validation Pipeline section for merge gate

[github-actions]

Component: Documentation

Bug Description:
The "PR Validation Pipeline" section in docs/architecture/workflows.md no longer accurately describes pr-validation.yml after PR #2213 introduced the pr-validation-success aggregator gate and gate-completeness-check job.

Specific stale content:

• Line 124 states the workflow "runs 16 parallel jobs" — the workflow now has 31 jobs (29 validation jobs + pr-validation-success gate + gate-completeness-check).
• Line 177 states "All jobs run in parallel with no dependencies" — this is incorrect now that pr-validation-success has a needs: list covering all 29 other jobs, and gate-completeness-check enforces that needs: list stays complete.
• The jobs table (lines 157–175) is missing the new gate and completeness-check jobs, and is also missing many of the 29 validation jobs added since this section was last written.
• The Mermaid flowchart does not reflect the aggregator gate structure.

Expected Behavior:
The section should:

• State the correct job count (31 jobs as of PR ci(workflows): add PR validation merge gate and fix docusaurus workflow #2213).
• Describe the pr-validation-success aggregator gate as the single required status check that gates merge, and gate-completeness-check as the job that enforces gate completeness.
• Update the jobs table to include all current jobs.
• Update or replace the Mermaid diagram to reflect the gate-based dependency structure.

Steps to Reproduce:
See PR #2213 (ci(workflows): add PR validation merge gate and fix docusaurus workflow) for the changes that introduced the gate.

Additional Context:

• Stale documentation: docs/architecture/workflows.md — PR Validation Pipeline section (lines ~122–177)

• Authoritative source: .github/workflows/pr-validation.yml and .github/workflows/README.md

• Reviewed and validated by a qualified human reviewer

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