fix: default seed direnv to warn

[sourishkrout]

Summary

• default zero-value seed direnv policy to enabled_warn
• keep explicit DirenvDisabled as the opt-out
• cover seed.Options{} and embedded LocalStoreOptions{} behavior

Verification

• go test ./internal/seed ./cmd -run 'Test(NewStore.*Direnv|DirenvPolicy|LocalStoreClient.*Direnv)' -count=1 -v
• make check
• runme run test
• runme run lint