feat: add service source command to configure repo source

[ndneighbor]

Summary

Adds a new railway service source command that allows configuring a service's repo source directly from the CLI, eliminating the need to use the dashboard for this operation.

Features

• Connect a GitHub repo to a service
• Set the branch (defaults to repo's default branch)
• Set root directory path within the repo
• Set watch paths/patterns to trigger deployments

Usage

railway service source --repo owner/repo [OPTIONS]

Options

Option	Description
--repo, -r	GitHub repo to connect (format: owner/repo) - required
--branch, -b	Branch name (optional, defaults to repo's default branch)
--root-directory	Root directory path within the repo
--watch-paths	Watch paths/patterns to trigger deployments
--service, -s	Service name or ID (defaults to linked service)
--environment, -e	Environment (defaults to linked environment)

Example

# Connect a service to a monorepo with root directory and watch paths
railway service source \
  --repo railwayapp/mono \
  --root-directory packages/central-station/apps/claude-code-runner \
  --watch-paths "packages/central-station/apps/claude-code-runner/**"

Changes

• Added ServiceConnect.graphql mutation for connecting a service to a source
• Added ServiceInstanceUpdateSource.graphql mutation for updating root directory and watch paths
• Added Source subcommand to service.rs with full implementation

[Milo123459]

You can already do this though the environment edit command, is it worth adding another command to do the same thing?