fix(cli): add missing Copilot variant to CliProviderType enum

[johntmyers]

Summary

• Adds the missing Copilot variant to the CliProviderType enum and its as_str() match arm, fixing --provider copilot CLI argument parsing.
• Adds a parity test (cli_provider_types_match_registry) that asserts CliProviderType variants match ProviderRegistry::known_types(), preventing this class of drift in the future.

Related Issue

Closes #707

Changes

• crates/openshell-cli/src/main.rs: Added Copilot variant to CliProviderType enum and "copilot" arm to as_str().
• crates/openshell-cli/src/main.rs (tests): Added cli_provider_types_match_registry test that iterates all CliProviderType variants via ValueEnum::value_variants() and compares against ProviderRegistry::known_types().

Testing

• cargo test -p openshell-cli --bin openshell cli_provider_types_match_registry passes.
• cargo fmt --check and cargo clippy clean on changed file.

Checklist

• Conventional commit format
• Pre-commit checks pass
• Tests added for regression prevention