fix(ci): guard auth gate package-mode builds

[NubsCarson]

Summary

Adds a small guard to the agent-review Auth P0 workflow so package-mode PR checkouts skip the local Eliza runtime plugin build when the nested eliza/packages/core checkout is absent.

Why

Several package-mode Milady PRs can fail before their actual auth tests because the pull_request_target workflow assumes eliza/packages/core exists and then runs:

(cd eliza/packages/core && bun run build)

That directory is only present in local-source/nested-Eliza checkouts. In package-mode checkouts the workflow should skip this optional local runtime plugin prebuild instead of failing with cd: eliza/packages/core: No such file or directory.

This is intentionally split from the larger Android APK/runtime PR so maintainers can land the base workflow fix independently.

Validation

git diff --check
node scripts/validate-ci-bootstrap-contract.mjs

Both passed locally.

Notes

Because this workflow runs under pull_request_target, GitHub evaluates the workflow file from the base branch. This PR may not be able to make its own Auth P0 run observe the guard until the change lands on develop; the value here is unblocking subsequent PR checks once merged.

Note

Skip local eliza runtime plugin build when eliza/packages/core is absent

Adds a directory existence check at the start of the 'Build local eliza runtime plugins' step in agent-review.yml. If eliza/packages/core is missing, the step prints a message and exits successfully instead of running build commands that would fail.

^{Macroscope summarized 5f777b4.}

[macroscopeapp]

Approvability

Verdict: Needs human review

Unable to check for correctness in 5f777b4.

^{You can customize Macroscope's approvability policy. Learn more.}

[github-actions]

Credit balance is too low

[NubsCarson]

Closing this standalone guard PR to avoid PR noise. The guard itself is still valid, but this branch cannot independently pass current package-mode CI from bare develop because the UI/package-mode fixes in #2153 are also required (typecheck fails on missing @elizaos/ui exports before this guard matters). The same Auth P0 guard remains included in #2153, where it is paired with the package-mode fixes that make the branch buildable.