Fix environment creation from shallow clones

[cyq1017]

Summary

• detect shallow repositories before creating an environment worktree branch
• fetch full history only when the source repo is shallow
• add a regression test that reproduces the shallow-clone push failure path

Fixes #248.

Tests

• go test ./repository -run TestInitializeWorktreeUnshallowOnShallowClone
• go test -short ./...
• git diff --check