docs(secret-scanning): document Agents secrets type

[happysnaker]

Summary

• add an Agents secrets section to the Understanding GitHub secret types reference page
• include scope, access, and restrictions for Copilot cloud agent secrets
• link to the dedicated cloud-agent setup guide from Further reading

Fixes #44913

[github-actions]

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Source	Review	Production	What Changed
code-security/reference/secret-security/secret-types.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Sharra-writes]

@happysnaker This issue hasn't yet been triaged and confirmed to need an update. Please refrain from opening PRs for issues that don't yet have a help wanted label, or else I'll have to block you from the repository for spam. Thanks.