feat(integrations): add remote provider integrations

[CalvinAllen]

Description

Add integrations with popular Git hosting providers to enhance the Git experience with rich remote information, avatars, PR/issue linking, and remote actions.

Scope Clarification: This issue covers the foundational integration layer - authentication, API access, and basic remote operations. Features that build on this foundation (like autolinks and PR management) have their own dedicated issues.

Supported Providers

Tier 1 (Full Support)

• GitHub (github.com)
• GitHub Enterprise Server
• Azure DevOps (dev.azure.com)
• Azure DevOps Server (on-premises)

Tier 2 (Standard Support)

• GitLab (gitlab.com)
• GitLab Self-Managed
• Bitbucket Cloud
• Bitbucket Server

Tier 3 (Basic Support)

• Gitea
• Gerrit

Integration Features

Authentication

• OAuth authentication flow
• Personal Access Token (PAT) support
• Secure credential storage
• Multiple account support

Enhanced Information

• Author avatars from provider
• Pull request information on commits
• Issue linking in commit messages
• Branch protection status
• CI/CD build status

Remote Actions

• Open file/commit/branch on remote
• Copy remote URLs
• Create pull request from branch
• View pull request details
• Open issues in browser

Configuration Options

• Provider-specific settings
• Custom domain support (Enterprise/Self-Managed)
• Authentication method preference
• Avatar caching settings
• Auto-link patterns per provider

Related Issues (Features that depend on this)

This issue provides the foundation for:

• feat(links): add autolink support for issues and external references #25 - Autolinks (uses provider APIs for enhanced issue/PR details in links)
• feat(pr): add Launchpad for pull request management #30 - Launchpad PR management (uses provider APIs for PR data)
• feat(views): add Contributors tool window #21 - Contributors view (uses provider APIs for avatars)

Note: Implement this issue first to enable the dependent features above. Basic functionality in dependent issues may work without full provider integration, but enhanced features require this foundation.

Acceptance Criteria

• Authentication works for each provider
• Avatars load correctly
• PR information displays on relevant commits
• Issue auto-linking works
• Remote URL operations function correctly
• Custom domains are supported