fix: complete persistent flags in shell completion

[sb123sb123]

Summary

Fixes #2173.

When shell completion runs for a subcommand, DefaultCompleteWithFlags passed only the command's local flags to the suggestion formatter. Persistent flags defined by an ancestor command are still valid after entering the subcommand, but they were omitted from suggestions, so a prefix such as --conf could not complete to --config.

This change combines visible persistent flags with the command's visible local flags before generating suggestions. The regression test covers a persistent string flag before and after a positional argument, plus the short - prefix used by zsh completion. Existing completion fixtures now mark their intentionally command-local flags with Local: true.

Testing

• go test . -count=1 -run TestCompletionIncludesPersistentFlagsAfterPositionalArgument
• go test . -count=1 -run TestPartialFlagCompletionAfterPositionalArgument
• go test . -count=1 -run TestDefaultCompleteWithFlags
• go test ./... -count=1
• go vet ./...
• gofmt checks and git diff --check

Limitations

• The initial dependency-resolution attempt could not reach proxy.golang.org; the successful test runs used the existing local module cache with GOPROXY=off.
• The race suite could not run on the Windows host: the default toolchain has cgo disabled, and enabling cgo failed because gcc is unavailable.
• golangci-lint, staticcheck, goimports, and gfmrun were not installed on the host.

AI assistance disclosure

This pull request was developed with assistance from OpenAI Codex. The submitting operator reviewed and understood the implementation, tests, verification results, and this description.

Release Notes

Fix shell completion of visible persistent flags on subcommands.