fix(codex): return remote auth refresh to external home

[Oldrich333]

Thinking Path

• Paperclip manages AI agents and their execution runtimes.
• The Codex adapter can run an agent on a remote SSH or sandbox target.
• An external CODEX_HOME can bind an agent to an independent Codex subscription and session store.
• A remote run stages that external home, but teardown sends every OAuth refresh to the shared host credential.
• The same-account guard protects the shared account, but the external account keeps stale OAuth data.
• This pull request returns the refresh to the credential source that supplied the remote run.
• The benefit is safe use of separate Codex subscription identities without an auth pool or a custom token manager.

Linked Issues or Issue Description

Fixes: #11094

Refs: #9621

Refs: #10005

Refs: #5952

What Changed

• Select the shared auth source for managed Codex homes.
• Select the external auth source for a remote run with an external CODEX_HOME.
• Resolve an external auth.json symlink before the atomic copy-back. This preserves the symlink.
• Keep the existing strictly-newer and same-account decision guard.
• Add end-to-end regression cases for regular and symlink-backed external auth sources.
• Document remote OAuth refresh ownership for external homes.

Verification

• pnpm exec vitest run packages/adapters/codex-local/src/server/execute.test.ts packages/adapters/codex-local/src/server/codex-auth-copyback.test.ts packages/adapters/codex-local/src/server/codex-home.test.ts
• Result: 3 test files passed. 66 tests passed.
• pnpm --filter @paperclipai/adapter-codex-local typecheck
• Result: passed.
• pnpm run check:tokens
• Result: passed.
• Independent review checked auth ownership, atomic writes, file mode, identity isolation, and token-safe logs.

Risks

Low risk.

The change only selects the copy-back destination for a remote run. Managed homes keep the existing shared destination. External homes already own their auth state. The existing decision predicate still refuses older data, unusable data, API-key data, and a different account identity.

No schema, API, migration, or local-run behavior changes.

Model Used

• OpenAI Codex gpt-5.6-sol with medium reasoning, tool use, and code execution produced the change. The harness did not disclose its context window size.
• Google Gemini gemini-3-pro-preview performed independent full reviews of the frozen commit.

Checklist

• I have included a thinking path that traces from project context to this change
• I have specified the model used (with version and capability details)
• I have checked ROADMAP.md and confirmed this PR does not duplicate planned core work
• I have searched GitHub for duplicate or related PRs and linked them above
• I have either (a) linked existing issues with Fixes: # / Closes # / Refs # OR (b) described the issue in-PR following the relevant issue template
• I have not referenced internal/instance-local Paperclip issues or links (only public GitHub #NNN / github.com/paperclipai/paperclip URLs)
• My branch name describes the change (e.g. docs/..., fix/...) and contains no internal Paperclip ticket id or instance-derived details
• I have run tests locally and they pass
• I have added or updated tests where applicable
• I have updated relevant documentation to reflect my changes
• I have considered and documented any risks above
• All Paperclip CI gates are green
• Greptile is 5/5 with no open P2s, recommendations, or follow-ups
• I will address all Greptile and reviewer comments before requesting merge