feat(git): support generic private repository hosts

[YogeLiu]

Summary

• add a transport-only git provider for unknown full HTTPS and SSH repository URLs
• preserve nested namespaces and delegate private-repository authentication to system Git credential helpers or SSH agents
• support clone, pull, and push while explicitly falling back to manual MR/PR creation for host-specific API operations
• extend provider configuration, remote import parsing, tests, and documentation

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature causing existing behavior to change)
• Documentation only
• Refactor / internal cleanup

Test Plan

• npm run typecheck passes
• npm test passes (1991 tests)
• npm run build passes
• Added/updated tests for provider detection, URL parsing, clone behavior, schema compatibility, and unsupported host API operations

Related Issues

Closes #300

Notes for Reviewers

Generic hosts intentionally do not attempt platform authentication, repository creation, or MR/PR APIs. Git remains the credential boundary, embedded HTTPS credentials are rejected, and branch push succeeds before the existing manual-PR fallback is shown.