docs: Update stale documentation - docs/planning/brds/sssc-planner-security-brd.md

## Stale Documentation Detected

**File:** `docs/planning/brds/sssc-planner-security-brd.md`
**Current ms.date:** `2026-06-13`
**Age:** 93 days (threshold: 90 days)

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**Workflow Run:** https://github.com/microsoft/hve-core/actions/runs/34827803501
**Detection Date:** 2026-09-14

### Action Required
- [ ] Review and update this documentation file
- [ ] Update `ms.date` frontmatter to current date
- [ ] Verify technical accuracy of content
- [ ] Close this issue manually after fixes are merged

Open docs/planning/brds/sssc-planner-security-brd.md and review its frontmatter and content for stale or inaccurate information. Update ms.date to the current date and verify the document's technical accuracy. Done means the updated documentation is merged and the issue is closed manually.

Written by the indexing model from the issue text.