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

[awdemos]

Endorsed for merge.

• Detects shallow user repos and runs before pushing the environment branch to the local container-use remote, fixing shallow-clone failures.
• Includes a regression test that constructs a shallow clone and verifies it becomes unshallow during environment creation.
• Verified: go test -short ./repository/... passes.

Minor note: if itself fails, the subsequent push retry will retry the push rather than reattempt unshallowing, but that's acceptable since the unshallow error will still surface.

[awdemos]

@cyq1017 friendly ping — this PR currently has merge conflicts with . Could you rebase onto and resolve the conflicts? Once the merge conflicts are gone I'll be happy to re-review.

[cyq1017]

Rebased onto the latest main and resolved the conflicts. I reran go test -short ./repository/... -count=1 and go test -short ./... -count=1; both pass. The PR is mergeable again and ready for re-review.