Add auth account store foundation

[cbusillo]

Summary

• add a codex-login auth_accounts store for auth_accounts.json with list/find/upsert/remove/set-active APIs
• persist active account id, timestamps, API-key and ChatGPT credential records with private file writes
• cover store behavior, profile coexistence, trailing JSON repair, claim-only ChatGPT account ids, active-id invariants, and fallback active-account promotion after deletion

Plan alignment

• Refs Port Every Code account store for multi-auth MVP #160: implements the account-store foundation slice only
• Refs Audit recent Codex Lab PRs against authoritative Every Code source #126: advances the audit-backed multi-auth lane without app-server, TUI, login-routing, or schema changes
• Leaves the follow-on login-flow upsert, profile import/discovery, and read-only /login account listing work in Port Every Code account store for multi-auth MVP #160

Validation

• just fmt
• just test -p codex-login auth_accounts (16/16)
• just test -p codex-login (155/155 passed; nextest reported one existing leaky test marker)
• cargo clippy -p codex-login --tests --no-deps -- -D warnings
• just fix -p codex-login
• git diff --check
• JetBrains inspection: inspect-closeout --scope changed_files GREEN on the exact worktree, fresh snapshot, 0 actionable findings

Review

• Gemini/Antigravity review: scope aligned; signed off on the active-account fallback fix with no issues
• GPT review: fixed ChatGPT account-id fallback matching and missing-active-id persistence behavior
• Auto-review: fixed the applicable account-store finding by promoting a remaining account when the active account is deleted; the separate agent_defaults.rs fallback-model note is outside this PR diff and remains separate audit work

Note: just fmt still reports the pre-existing uv parse warnings for exclude-newer = "7 days" in Python pyproject files.