chore(deps): update security dependencies

[appleboy]

Summary

• Upgrade Go and vulnerable transitive modules to patched releases.
• Migrate credential storage from the unavailable Authgate SDK module to the public Signet SDK.

Related issues

• GitHub: N/A

AI authorship

• AI was used
  • Tool / model: Codex (GPT-5)
  • AI-authored files: go.mod, go.sum, util/credstore.go, util/credstore_test.go
  • Human line-by-line reviewed: None — not yet reviewed by a human.

Change classification

• Leaf change

Verification

• Automated: make lint; make test; git diff --check
• Manual: Confirmed the Signet SDK provides the credential-store API used by CodeGPT.

Security check

• No secrets in the diff
• N/A - no external interface changed

Risk and rollback

• Risk: Credential-store behavior depends on the SDK-compatible API.
• Rollback: Revert commits 4dbc79b and 346bfab.

Reviewer guide

• Read carefully: util/credstore.go and module replacement.

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