feat(builtins): add compgen command

[chaliy]

Summary

compgen generates possible completions for a word. While primarily known for interactive tab-completion, it's actually useful in scripts for word matching, filtering, and variable/function introspection.

Requested Flags

Flag	Description
-W wordlist	Match against the given word list
-v	Match variable names
-A function	Match function names
-A builtin	Match builtin command names
-A command	Match all command names (builtins + functions)
-A variable	Match variable names (same as -v)
-A file	Match filenames in current directory
-A directory	Match directory names
-P prefix	Add prefix to each match
-S suffix	Add suffix to each match

Use Cases

# Filter words by prefix
compgen -W "start stop restart reload" -- "re"
# Output: restart, reload

# List all variables starting with "BASH"
compgen -v BASH
# Output: BASH_VERSION, BASHPID, BASH_REMATCH, ...

# List all defined functions
compgen -A function

# List all builtins
compgen -A builtin

# Check if a command exists
if compgen -A command -- "git" > /dev/null; then
    echo "git is available"
fi

# Build dynamic menus
options=$(compgen -W "$available_commands" -- "$user_input")

Implementation Notes

• Does NOT require interactive shell or readline
• Should query interpreter's variable store, function table, and builtin registry
• -A file and -A directory should query VFS
• Each match printed on separate line to stdout
• Exit 0 if matches found, 1 if no matches
• complete builtin (companion) is lower priority — compgen is the scriptable one

[chaliy]

Resolved in PR #585 or #588