Related to #1456

When GHE host environment variables are set, provider add / TUI / device-code login use the configured enterprise GitHub host and API base for OAuth and Copilot token exchange. Without those variables, the provider keeps the existing github.com defaults, so standard GitHub Copilot users remain on the current flow.

Validated

• GHE device-code login and live Copilot chat path
• Default github.com endpoint behavior via focused regression tests
• Provider CLI/TUI configuration paths
• Copilot request header contract and token exchange behavior
• dotnet build Netclaw.slnx
• dotnet slopwatch analyze (reports existing unrelated timing-test warnings)
• ./scripts/Add-FileHeaders.ps1 -Verify

Not included

• No change to the default GitHub Copilot host when GHE env vars/options are absent
• No persisted enterprise secrets; environment-token mode reads COPILOT_GITHUB_TOKEN / GH_TOKEN / GITHUB_TOKEN at runtime

Tests:
With my GHE tenant env set TUI behavior:

Requests device login flow for the tenant:

Browser login:

Status netclaw status with daemon running:

.\Netclaw.exe status
overall: healthy
version: 0.24.2 (commit 68f0a7a, built 2026-06-25T14:36:18Z)
daemon: PID 47508, uptime 0m 15s, endpoint http://127.0.0.1:5199
persistence: Sqlite
memory: sqlite (healthy)
telemetry: disabled
model: gpt-5-mini (provider: github-copilot, context: 400,000 tokens)
  input: Text, Image
  output: Text
connectors:
- discord: disabled (disabled)
    Discord connector is disabled in configuration.
- mattermost: disabled (disabled)
    Mattermost connector is disabled in configuration.
- slack: disabled (disabled)
    Slack connector is disabled in configuration.
- tui: healthy (enabled)

Chat test: