docs: add required Security Planner cadence conformance job to PR validation pipeline docs

### Component

Documentation

### Bug Description

PR #2875 (commit 68ab2498) added a new required job, `security-planner-cadence-conformance`, to `.github/workflows/pr-validation.yml`. The job calls the reusable `agent-conformance.yml` workflow (now parameterized with a `suites` input) scoped to just `agent-conformance-security-planner`, and is listed as a dependency of the pipeline's final required-checks aggregation job.

`docs/architecture/workflows.md` documents the PR Validation Pipeline with a Mermaid diagram and a "### Jobs" table (currently listing spell-check, markdown-lint, table-format, psscriptanalyzer, yaml-lint, pester-tests, frontmatter-validation, skill-validation, link-lang-check, markdown-link-check, dependency-pinning-check, devcontainer-lockfile-check, npm-audit, codeql, copyright-headers, plugin-validation, gitleaks-scan). This table does not include the new `security-planner-cadence-conformance` job (or the pre-existing `eval-validation` job), so the documented job inventory for the pipeline is incomplete relative to the current workflow.

### Expected Behavior

The Jobs table (and diagram, if scoped to include analysis/testing jobs) in `docs/architecture/workflows.md` should list `security-planner-cadence-conformance` with its reusable workflow (`agent-conformance.yml`, scoped via the `suites` input to `agent-conformance-security-planner`) and what it validates (Security Planner disclaimer cadence conformance), so the documented pipeline matches the actual required checks in `pr-validation.yml`.

### Steps to Reproduce

Compare the `jobs:` section of `.github/workflows/pr-validation.yml` at commit 68ab2498 (PR #2875) against the "### Jobs" table in `docs/architecture/workflows.md`.

### Additional Context

Documentation: `docs/architecture/workflows.md` (PR Validation Pipeline section).

Code: `.github/workflows/pr-validation.yml`, `.github/workflows/agent-conformance.yml`.

> Generated by [Documentation Update Check](https://github.com/microsoft/hve-core/actions/runs/34413691174) · auto · 70.5 AIC · ⌖ 2.01 AIC · ⊞ 10K · [◷](https://github.com/search?q=repo%3Amicrosoft%2Fhve-core+is%3Aissue+%22gh-aw-workflow-call-id%3A+microsoft%2Fhve-core%2Fdoc-update-check%22&type=issues)

Start with the Jobs table and PR Validation Pipeline diagram in docs/architecture/workflows.md, then compare them with the jobs section of .github/workflows/pr-validation.yml at commit 68ab2498. Check .github/workflows/agent-conformance.yml for the reusable workflow and suites input. Done means the documentation includes security-planner-cadence-conformance, its scope and validation purpose, and accounts for the existing eval-validation job.

Written by the indexing model from the issue text.