feat: add --callback-port flag to login command

[dsdshcym]

Summary

• Add --callback-port <port> option to mcpc login to specify the OAuth callback port
• Some OAuth providers (e.g., Slack MCP) register specific redirect URIs with fixed ports — the hardcoded port 8000 causes redirect_uri mismatch errors
• When --callback-port is provided, it's used directly instead of auto-selecting from port 8000

Motivation

Slack's MCP server (mcp.slack.com/mcp) uses client ID 1601185624273.8899143856786 with a registered redirect URI on port 3118. Without this flag, mcpc login always starts the callback server on port 8000, which Slack rejects:

redirect_uri did not match any configured URIs. Passed URI: http://localhost:8000/callback

With this fix:

mcpc login mcp.slack.com/mcp --client-id "1601185624273.8899143856786" --callback-port 3118

Test plan

• mcpc login --help shows the new --callback-port option
• mcpc login <server> --client-id <id> --callback-port 3118 uses port 3118 for the callback
• Without --callback-port, behavior is unchanged (auto-selects from 8000)
• Tested successfully against Slack MCP OAuth flow

🤖 Generated with Claude Code