Fix env overrides when config file doesn't exist

[Copilot]

config.Load() returned early when the config file was missing, skipping environment variable overrides. This prevented env-only configuration (e.g., JIRA_API_TOKEN, JIRA_URL in CI/CD).

Changes:

• Call applyEnvOverrides() before returning when os.IsNotExist(err) in Load()
• Add test coverage for env-only configuration without config file

Before:

data, err := os.ReadFile(path)
if os.IsNotExist(err) {
    return cfg, nil  // env vars ignored
}

After:

data, err := os.ReadFile(path)
if os.IsNotExist(err) {
    cfg.applyEnvOverrides()
    return cfg, nil
}

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