Use global Decodex account pool

[yvette-carlisle]

Summary

• Move Codex account pool resolution to global ~/.codex/decodex/accounts.jsonl
• Remove project-local codex.accounts.path support and update fixtures/docs
• Add account rotation persistence and proactive token refresh

Verification

• cargo test -p decodex codex_accounts --lib
• cargo make fmt-check
• cargo make check-rust
• cargo make check-vstyle-rust
• cargo make test-rust
• decodex project list
• decodex status --json