feat: support xor/and groups for positional arguments

[toller892]

Fixes #255

Problem

xor and and group validation only worked for flags, not positional arguments. Providing both a flag and a positional arg in the same xor group did not produce an error.

Changes

• context.go: Extended checkMissingFlags, checkXorDuplicates, and checkAndMissing to include positional arguments in xor/and group validation
• kong.go: Extended checkOverlappingXorAnd to also scan positional arguments for xor/and group membership
• Error messages now use ShortSummary() instead of hardcoded --%s format, so they correctly display both flag names (--flag) and positional arg names

Example

type CLI struct {
    Defaults bool     `xor:"group1" help:"Use defaults"`
    Args     []string `arg:"" xor:"group1" optional:"" help:"Positional args"`
}

Before: run --defaults foo bar → no error
After: run --defaults foo bar → error: "--defaults and args can't be used together"

Tests

All existing tests pass. The fix is backward-compatible since positional args without xor/and tags are unaffected.

[alecthomas]

Please add tests.

[toller892]

Tests added! I've added 3 new test cases covering xor/and with positional arguments:

• TestXorWithPositional — flag vs positional in same xor group (all 4 combinations: both, flag-only, positional-only, neither)
• TestXorFlagAndPositionalInverted — same test with reversed field order
• TestAndWithPositional — flag + positional in same and group (missing positional detected)

The implementation was also improved: the xor/and validation now groups paths by command scope (root vs child commands) to correctly handle both flat structs and subcommands. All 2700+ existing tests pass.