Show default login identity in picker

[shiny-code-bot]

Summary

• show the default /login row identity using stored auth metadata when available
• add AuthDotJson helpers for account email/id so TUI does not duplicate token or agent-identity parsing
• cover ChatGPT default picker rendering and agent-identity metadata extraction

Tests

• cargo test -p codex-login auth_dot_json_exposes_agent_identity_account_metadata
• cargo test -p codex-tui login_slash_command_opens_profile_picker
• cargo check -p codex-tui
• cargo fmt --package codex-login --package codex-tui -- --check

Review

• review agent flagged agent-identity default accounts as a gap; fixed by moving identity extraction into codex-login