feat: delete local tracking branches when deleting environments

[zerone0x]

Summary

• When container-use delete removes an environment, it now also cleans up any local branches in the user's repo that were tracking the environment's remote branch (e.g., cu-<env-id> created by container-use checkout)
• Uses git for-each-ref to find branches tracking container-use/<id> and deletes them automatically
• Safely skips deletion if the branch is currently checked out, with a warning message

Closes #335

Test plan

• go build ./... passes
• go test -short ./... passes (pre-existing failures unrelated to this change)
• go vet ./... passes
• Manual test: create environment, checkout branch, delete environment, verify local branch is removed

🤖 Generated with Claude Code

[awdemos]

Endorsed for merge.

• Cleans up local tracking branches in the user repo when deleting an environment, preventing stale cu- branches from accumulating.
• Safely skips the currently checked-out branch with a warning rather than failing.
• Verified: go test -short ./repository/... passes.