[WRONG BRANCH] fix(oauth): preserve legacy identity-less accounts on forced add-account login

[luvs01]

Motivation

• Prevent a newly identified OAuth credential from overwriting an active legacy identity-less account when the operator intentionally performs a forced add-account login, which previously caused data loss for providers like Kimi.

Description

• Change runLogin to pass preserveIdentityless: opts?.forceLogin === true into saveCredential so forced add-account logins preserve an existing identity-less active slot for any provider instead of only for kiro (src/oauth/index.ts).
• Add a focused regression test asserting that a forced Kimi add-account appends the identified account while keeping the legacy identity-less account (tests/oauth-reauth-bind.test.ts).
• No user-visible configuration or auth contract changes were made.

Testing

• Ran the focused tests tests/oauth-reauth-bind.test.ts and tests/kimi-oauth-identity.test.ts, and both passed.
• Ran bun run typecheck and bun run privacy:scan, and both succeeded.
• Attempted a full bun run test; many tests passed in this environment, but unrelated Grok model-discovery tests timed out and GUI dependency downloads returned HTTP 403, so the full-suite run is environment-limited rather than indicative of the change's correctness.

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Codex Task

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✅ Deterministic PR hygiene checks passed.

[github-actions]

⏳ DRAFT

• wrong target branch (main); retarget to dev.

What to do

• Retarget this PR to dev — all contributions go to dev.

Its title has been prefixed with [WRONG BRANCH].
This pull request was already a draft. Its draft status will be preserved after every issue above is resolved.

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