fix(cli): missing Copilot variant in CliProviderType enum

[htekdev]

Summary

PR #476 added Copilot as a supported provider (copilot.rs, network policy, credential discovery) but missed adding the Copilot variant to the CliProviderType enum in crates/openshell-cli/src/main.rs. This causes --provider copilot to fail at CLI argument parsing.

Reproduction

openshell sandbox create --provider copilot -- copilot -sp "Hello"
# Error: CLI does not recognize "copilot" as a valid provider type

Fix

Add Copilot to the CliProviderType enum in crates/openshell-cli/src/main.rs, matching the pattern used by other providers (Claude, Codex, OpenCode, etc.).

Single commit fix available at:
main...htekdev:OpenShell:fix/copilot-cli-enum

Agent Diagnostic

Traced the issue from CLI argument rejection to the missing enum variant. Built locally and confirmed --provider copilot works after the fix.

Related

• PR feat(providers): add GitHub Copilot CLI agent provider #476 (merged) — Initial Copilot provider support (missing this enum variant)