[codex] improve test coverage and add CI gate

[danjdewhurst]

Summary

• adds focused client and server tests to raise coverage above the 80% adjusted threshold
• adds a reusable coverage check script that counts unmeasured source files against the gate
• adds a GitHub Actions workflow for pushes and pull requests targeting main

Validation

• bun run test:coverage
• bun run coverage:check
• bun run typecheck
• biome check on changed files

Notes

• full repo lint still includes pre-existing generated dist formatting issues outside this change