feat(agents): allow granted direct creation

[JaviBTechi]

Summary

• add buzz agents create as an owner-authorized, Desktop-mediated direct creation path
• add per-agent, per-owner revocable grants with source-channel attachment
• persist request IDs for crash-safe retries and return owner-signed completion evidence
• capture relay and signer scope across creation, membership, and start operations

Related issue

Related: #4869

Testing

• focused direct-creation tests: 18 passed
• Desktop TypeScript typecheck: passed
• Biome checks: passed
• file-size and security guards: passed
• Rust formatting: passed
• full Desktop suite after rebase: 5,891 passed, 1 failed in src/shared/lib/useDocumentVisible.test.mjs; that file and its implementation are unchanged from main
• Rust tests require CI because this Windows host does not have the MSVC link.exe toolchain

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 1c8321cd08feb597f8bcff5195c21148fb3e98ed...9fd3196499b11d922bc28ecb5104f0fd3fe05a27.
A new review must complete for this exact range. When manual authorization
is required, a Block organization member must comment exactly
@buzz-security-review 9fd3196499b11d922bc28ecb5104f0fd3fe05a27 to authorize a new review.
Any previous review applies only to its recorded range.