Upgrade google/go-github from v29 to v63+

[vedantlavale]

Description:

The project uses deprecated github.com/google/go-github/v29 (go.mod:35) which is no longer maintained.

Current:

github.com/google/go-github/v29 v29.0.3

Latest:

github.com/google/go-github/v67 v67.0.0

Why this matters:

• v29 is no longer maintained (security patches, bug fixes)
• New features not available (GitHub API rate limit helpers, webhook signatures)
• Breaking changes accumulate, harder to upgrade later

Suggested fix:

1. Update import path in all files using github.com/google/go-github/v29 → github.com/google/go-github/v67
2. Fix any API breaking changes (common ones: context usage, response parsing)
3. Update go.mod with go get github.com/google/go-github/v67
4. Run go mod tidy
   Files likely affected:

• pkg/plugin/sdk/client.go
• tool/actions-plan-preview/main.go
• Any other GitHub API callers

[vedantlavale]

I'd like to work on this. Happy to submit a PR for this dependency upgrade.