[WRONG BRANCH] oauth: remove legacy auth backup on destructive mutations (logout/account delete)

[luvs01]

Motivation

• Prevent legacy single-credential backups (auth.json.pre-multiauth) from retaining OAuth refresh tokens after destructive user actions like logout or account deletion.

Description

• Add removeLegacyBackup() and make mutateStore() accept options.removeLegacyBackup so callers can opt to delete the one-time legacy backup after a destructive mutation.
• Only create the downgrade backup on the first non-destructive migration; destructive mutations (logout and account deletion) invoke the new removal path so auth.json.pre-multiauth is removed after the store is persisted.
• Wire the removal behavior into both removeCredential() and removeAccount() so active-credential logout and explicit account deletion clear the legacy backup.
• Add regression tests to tests/oauth-store-multi.test.ts verifying that a logout and an account deletion do not leave auth.json.pre-multiauth behind while preserving the existing non-destructive migration test.

Testing

• Ran the focused auth-store tests with bun test tests/oauth-store-multi.test.ts and they passed (20 passed, 0 failed).
• Ran type checking with bun run typecheck and bun run privacy:scan, both succeeded.
• Ran the broader bun run test during validation; the focused OAuth tests remained green but the full suite hit unrelated timeouts in other tests (observed in oauth-login-cli-live-update.test.ts and management-provider-validation.test.ts) while exercising the whole test matrix.

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📥 Commits

Reviewing files that changed from the base of the PR and between 57140d6 and e7a754d.

📒 Files selected for processing (2)

• src/oauth/store.ts
• tests/oauth-store-multi.test.ts

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✅ Deterministic PR hygiene checks passed.

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⏳ 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].
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