feat: add --log-level to control CLI logging

[Spidy092]

Summary

Add --log-level so commands such as openserp search bing "golang" --log-level error can suppress warnings, including the warning emitted when no default config file exists.

The setting also supports app.log_level and OPENSERP_APP_LOG_LEVEL, with CLI > environment > config precedence. A non-empty value overrides the logging threshold selected by quiet/verbose/debug; an empty value retains existing defaults. Existing logger output destinations and browser settings are preserved.

Closes #44.

Why

The missing-config warning was logged before normal logger initialization. The merged log level is now applied before startup warnings and reapplied after logger setup. Invalid levels return a configuration error before search starts.

Testing

• make fmt
• make test (full suite with race detector)
• make lint (go vet and golangci-lint v1.64.8)
• go build
• npx --yes @redocly/cli lint docs/openapi.yaml (passes with 9 warnings in the unchanged specification)
• Regression tests for config/environment/CLI precedence, startup warnings, invalid values, explicit missing config paths, and legacy logging flag interactions.
• Built CLI checks for help, warning suppression, environment overrides, and errors remaining visible on stderr. Used the unsupported Bing raw-mode path to avoid browser/network requests.

Live engine integration tests were not run; the change is confined to CLI logging configuration.

Checklist

• Linked the related issue.
• Updated README and example configuration.
• Kept tests deterministic and independent of browsers/search engines.
• Kept the change focused on logging configuration.