Warn when config contains bad URL

[arbourd]

Prints a warning to stderr when a user Provider contains a malformed and non-functional URL. Adds tests for DefaultProviders. All errors and warnings are now piped to stderr over stdout.

[Copilot]

Pull request overview

This PR adds validation around custom provider URLs loaded from global git config and updates CLI error output to go to stderr.

Changes:

• Preserve deterministic provider iteration order when loading providers from git config.
• Emit a warning and skip entries when a provider URL from git config is invalid (parse fails or missing host).
• Add tests to validate default provider BaseURLs and extend provider loading tests for invalid URLs; print CLI errors to stderr.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
open/provider.go	Track provider insertion order and warn/skip invalid provider BaseURLs from git config.
open/provider_test.go	Add BaseURL validity test for defaults; add invalid-url case and minor parsing refactor in LoadProviders tests.
main.go	Route error messages to stderr instead of stdout.

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