chore(ci): re-enable codecov fail_ci_if_error after Codecov registration

[cjimti]

`ci.yml` currently sets `fail_ci_if_error: false` on the Codecov upload step because the repository is not yet registered with Codecov, and a hard "Repository not found" failure was blocking PRs. Once the repo is onboarded at https://app.codecov.io/gh/plexara/plexara-agents:

1. Confirm `CODECOV_TOKEN` is set on the repo (`gh secret list --repo plexara/plexara-agents`).
2. Confirm a successful upload for at least one PR.
3. Flip `.github/workflows/ci.yml` Codecov step to `fail_ci_if_error: true`.
4. Add a test PR to confirm the gate now blocks merges on legitimate upload failures.

Background: PR #11 review comment (D5) flagged that the `false` setting masks future regressions until someone remembers to flip it.