feat: support OAuth connections for GitHub and GitHub Enterprise

[tomkis]

Summary

• Users need to connect both GitHub.com and GitHub Enterprise (GHE) accounts simultaneously
• Once connected, agents should be able to access repos on either host transparently

Requirements

• User can initiate OAuth for github.com and a configured GHE instance independently
• Both connections stored as separate per-user secrets in OneCLI (keyed by hostname)
• Credential gateway injects the correct token based on request target host
• UI shows connection status for each GitHub provider, with connect/disconnect per provider
• Agent pods can access both hosts when both are connected (network policy allows both)

[jjeliga]

onecli enabler: kagenti/onecli#5

[jjeliga]

onecli enabler followup: kagenti/onecli#20