feat(accounts): export OpenCode auth json

[Komzpa]

Summary

Revives #532 on a maintainer-owned branch because the original PR head has maintainerCanModify=false.

Credit: original implementation by @lNimien in #532; this branch rebases and updates that work on current main.

• Adds OpenCode auth JSON export flow for accounts.
• Fixes the known review blocker by exporting chatgpt_account_id directly, so unknown real ChatGPT account ids serialize as null instead of falling back to the local codex-lb account id.

Validation

• ruff check app/modules/accounts app/core/auth tests/integration/test_account_opencode_auth_export.py passed
• ruff format --check app/modules/accounts app/core/auth tests/integration/test_account_opencode_auth_export.py passed
• git diff --check origin/main...HEAD passed
• pytest tests/integration/test_account_opencode_auth_export.py was blocked in this isolated worktree because no .venv exists and system pytest is missing pydantic_settings
• ty check ... was blocked because ty is not installed on PATH and no .venv/bin/ty exists

OpenSpec

• OpenCode auth export requirements remain included and updated.

Revives #532

[chatgpt-codex-connector]

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Reviewed commit: 42f8cc818d

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[lNimien]

Hey, thanks for reviving this PR.

Sorry I was offline for about a week due to holidays, so I couldn’t respond earlier on #532. I’ve now enabled “Allow edits by maintainers” for future PRs.

Since #757 is based on my original implementation from #532, could you please keep the attribution/authorship clear in the final merge? I’m also happy to review this PR or help with any remaining fixes.

[Soju06]

@codex review

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Codex Review: Didn't find any major issues. What shall we delve into next?

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[Soju06]

@codex review

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Codex Review: Didn't find any major issues. 🎉

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[Soju06]

@codex review

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Codex Review: Didn't find any major issues. Already looking forward to the next diff.

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[Soju06]

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