fix(portal): show agent version build timestamp

[mnkj0021]

Fixes #675

Summary

• show each deployed agent version's actual buildTime in the timestamp on the right
• keep the displayed timestamp consistent with the build metadata shown in the same row

Root cause

The version row displayed version.createdAt on the right while the build metadata displayed version.buildTime. Those fields can differ, so the UI could show two conflicting dates for the same deployed version.

Validation

• corepack pnpm --filter portal build — passed on a fresh recheck
• git diff --check — passed
• branch is current with upstream main (0 commits behind)

GitHub's upstream CI workflows are still in action_required and need a Scope maintainer to approve the fork workflow run. The Microsoft CLA check passes.

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