feat: add Codex access token accounts

[zakyirsyaad]

Summary

• add K12 Codex access-token account support using codex login --with-access-token
• fetch K12 usage through AgentIdentity AgentAssertion auth
• clarify the add-account modal copy for K12 tokens
• improve Codex CLI resolution for GUI-launched apps and preserve PATH for npm/node shims
• harden account/auth persistence with atomic writes and serialized store mutations
• add AGENTS.md contributor guidelines for this repository

Test plan

• cargo test --manifest-path src-tauri/Cargo.toml
• pnpm build
• local K12 usage smoke test returned plan_type k12 with no usage error