fix: Improve Docker/server support and fix glab --jq compatibility

[konard]

Summary

This PR addresses the issues reported in #5 about glab-setup-git-identity not working properly in Docker/server environments:

• Fixed README.md manual commands to use pipe to jq instead of --jq flag (glab api does not have a built-in --jq flag in all versions)
• Added note about jq requirement for manual commands
• Added "Authentication in Docker/Server Environments" section to README with detailed instructions for:
  • Using screen + curl -L to complete OAuth in Docker containers
  • Using SSH port forwarding for remote servers
  • Token-based authentication (recommended for CI/CD)
• Enhanced CLI with helpful headless auth instructions - when authentication is needed in Docker/server environments, the tool now prints clear instructions on how to complete the OAuth flow
• Fixed src/index.js to parse JSON responses in JavaScript instead of relying on glab's --jq flag, improving compatibility with all glab versions
• Optimized getGitLabUserInfo to use a single API call instead of two

Changes

README.md

• Fixed example commands to use glab api user | jq -r '.username' instead of glab api user --jq '.username'
• Added note that jq must be installed for manual commands
• Added comprehensive section on authentication in headless environments
• Fixed manual auth command from glab auth login --hostname... --git-protocol... to glab auth login (interactive mode doesn't support extra flags)

src/cli.js

• Added printHeadlessAuthInstructions() function that displays helpful instructions when authentication is needed
• Updated handleNotAuthenticated() to show these instructions before attempting auth
• Improved error messages with multiple options for authentication

src/index.js

• getGitLabUsername() now parses JSON response in JavaScript instead of using --jq flag
• getGitLabEmail() now parses JSON response in JavaScript instead of using --jq flag
• getGitLabUserInfo() optimized to make a single API call and parse both values
• Added better error messages for parsing failures

Test Plan

• All 13 unit tests pass
• ESLint passes with no warnings
• Prettier formatting verified
• jscpd duplication check passes
• CI checks pass

Fixes #5

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[konard]

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