docs: update security script documentation for Get-CodeScanningAlerts.ps1

**Component:** Documentation

## Bug Description

`scripts/security/Get-CodeScanningAlerts.ps1` was added in #1418 (feat: add github-security code-scanning skill) but neither `scripts/README.md` nor `scripts/security/README.md` was updated to document it.

The script is distributed as a symlink from `scripts/security/Get-CodeScanningAlerts.ps1` → `.github/skills/github/gh-code-scanning/scripts/Get-CodeScanningAlerts.ps1`.

**`scripts/README.md` — Security Scripts table** lists four scripts but omits `Get-CodeScanningAlerts.ps1`:

```
| Test-DependencyPinning.ps1 | Validate dependency pinning compliance |
| Test-SHAStaleness.ps1 | Check for outdated SHA pins |
| Update-ActionSHAPinning.ps1 | Automate updating GitHub Actions SHA pins |
| Test-ActionVersionConsistency.ps1 | Validate action version consistency |
```

**`scripts/security/README.md` — Scripts section** has individual `###` subsections for each script but has no section for `Get-CodeScanningAlerts.ps1`.

## Expected Behavior

- `scripts/README.md` should include a row for `Get-CodeScanningAlerts.ps1` in the Security Scripts table (e.g., `Retrieve and group GitHub code scanning alerts via the gh CLI`).
- `scripts/security/README.md` should have a `### Get-CodeScanningAlerts.ps1` section documenting its purpose, parameters (`-Owner`, `-Repo`, `-Branch`, `-OutputFormat`), and usage examples, consistent with the pattern used for other scripts in that file.
- The symlink relationship between `scripts/security/Get-CodeScanningAlerts.ps1` and `.github/skills/github/gh-code-scanning/scripts/Get-CodeScanningAlerts.ps1` should be noted.

## Steps to Reproduce

Triggered by commit `5bedf80` — PR #1418 merged to main on 2026-04-30.

## Additional Context

- Script file: [`scripts/security/Get-CodeScanningAlerts.ps1`](scripts/security/Get-CodeScanningAlerts.ps1) (symlink)
- Canonical source: `.github/skills/github/gh-code-scanning/scripts/Get-CodeScanningAlerts.ps1`
- Documentation files requiring update: [`scripts/README.md`](scripts/README.md), [`scripts/security/README.md`](scripts/security/README.md)
- Related open issues for the same README: #1434, #1435

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Read scripts/README.md and scripts/security/README.md, then inspect .github/skills/github/gh-code-scanning/scripts/Get-CodeScanningAlerts.ps1 for its parameters and usage. Add the missing security-table row and a matching script section with examples, purpose, parameters, and the symlink relationship; done means both README files document the script consistently.

Written by the indexing model from the issue text.