Support option completion for default subcommands

[pentaoa]

Adds a default subcommand's named arguments to its parent command's generated completions, so options and flags remain completable when the default subcommand name is omitted.

The completion metadata retains the owning subcommand for custom completion callbacks and removes duplicate option names inherited through nested defaults. Positional arguments are intentionally not merged because they would compete with explicit subcommand names.

Fixes #488.

Checklist

• I've added tests that validate Bash, Zsh, and Fish output
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the existing math completion snapshots

Testing

• swift build --product ArgumentParser
• swift format lint --strict on all changed Swift files
• generated Bash, Zsh, and Fish scripts for the existing math example match the updated snapshots
• manually exercised Bash completion for the omitted default subcommand and an explicit sibling subcommand

The full Swift test target could not run locally because this host only has Command Line Tools and its SDK does not provide XCTest; the repository CI matrix will run the added tests.

AI assistance: Codex was used to investigate the issue, implement the change, and draft tests and this PR description. I reviewed the diff and verified the behavior described above.