fix(tui): guard local.agent.current() after oauth redirect (fixes #7727)

[cgarrot]

Fixes #7727

• TUI: guard agent access while the agent list is temporarily unavailable after OAuth redirect (prevents local.agent.current().name crash).
• Auth: persist OAuth extra metadata returned by auth plugins (accountId, enterpriseUrl) + add regression test.

Verified:

• bun run typecheck (packages/opencode)
• bun test test/provider/auth-extra-fields.test.ts

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found:

• PR fix(tui): handle undefined agent.current() to prevent fatal TypeError #7689: "fix(tui): handle undefined agent.current() to prevent fatal TypeError"
  • URL: fix(tui): handle undefined agent.current() to prevent fatal TypeError #7689
  • Why related: Both PRs address the same core issue - preventing crashes when agent.current() is undefined/unavailable in the TUI. PR fix(tui): handle undefined agent.current() to prevent fatal TypeError #7689 appears to be an earlier attempt to fix the undefined agent access problem, while PR fix(tui): guard local.agent.current() after oauth redirect (fixes #7727) #7747 targets the specific scenario after OAuth redirect.