docs: add cgt audit report (#18328)

* docs: add cgt audit report

* Update docs/security-reviews/README.md

Co-authored-by: Matt Solomon <[email protected]>

---------

Co-authored-by: Matt Solomon <[email protected]>

Author: agusduha

docs/security-reviews/2025_11-Custom-Gas-Token-Spearbit.pdf

@@ -46,6 +46,7 @@ Please see the report for the specific details.
 | 2025-07 | Spearbit                   | VerifyOPCM                                                                                                                                                                                                                | [2025_07-VerifyOPCM-Spearbit.pdf](./2025_07-VerifyOPCM-Spearbit.pdf) / [Auditor hosted report][SpearbitJuly25]                                             | 731280c6fc0ad184d252e0fb1d0ad12b5f59fd60 | op-contracts/v4.0.0 |
 | 2025-09 | Spearbit                   | U16a                                                                                                                                                                                                                      | [2025_09-U16a-Spearbit.pdf](./2025_09-U16a-Spearbit.pdf)                                                                                                   | 475801690f7a451469ee4da87b5fe3c54c92f372 | op-contracts/v4.1.0 |
 | 2025-10 | Spearbit                   | U17                                                                                                                                                                                                                       | [2025_10-U17-Spearbit.pdf](./2025_10-U17-Spearbit.pdf)                                                                                                     | aeed7033f7f739d8ecd4bd70a42ff09013bbc91e | op-contracts/v5.0.0 |
+| 2025-11 | Spearbit                   | Custom Gas Token                                                                                                                                                                                                          | [2025_11-Custom-Gas-Token-Spearbit.pdf](./2025_11-Custom-Gas-Token-Spearbit.pdf)                                                                           | 1f888ede1940fce20f71db89fc13039fdd96757e | op-contracts/v6.0.0 |
 
 <!-- Note: in order to reduce dependency on the honesty of repo maintainers, we should include a link to the auditor hosted report whenever possible. -->