fix: honour -- as end-of-options for file-operand commands

[davydog187]

Fixes #83

POSIX -- ends option parsing; everything after it is an operand. Some commands already did that (sort, wc, head, tac, rev). Others treated -- as a filename or rejected it as an invalid option.

The worst case was split-brain: shasum -- /f printed the correct hash and exited 1 with shasum: --: No such file or directory, so a caller checking the exit code and a caller reading stdout disagreed about whether it worked.

User-visible behavior

# before
$ shasum -- /f
<hash>  /f
shasum: --: No such file or directory    # rc=1, and the correct hash on stdout
$ cat -- /f
cat: --: No such file or directory       # rc=1
$ md5sum -- /f
md5sum: invalid option '--'              # rc=1
$ base64 -- /f
base64: invalid option '--'              # rc=1

# after (matches GNU/BSD)
$ shasum -- /f
<hash>  /f                               # rc=0, stderr empty
$ cat -- /f
hello
$ md5sum -- /f
<hash>  /f                               # rc=0
$ base64 -- /f
aGVsbG8K                                 # rc=0

-- is also how a filename beginning with a dash is passed: cat -- -f reads a file named -f. JustBash.exec/2 does not raise.

Changes

• StdinOperand.split_end_of_options/1 is the shared -- stop that FlagParser and StdinOperand.operands/1 already implemented. Hand-rolled parsers call it once at their entry point, parse flags from the left side, and append the remainder as operands.
• Commands that take no options (cat, touch, mv, source) drop -- rather than opening a path named --.
• test/commands/end_of_options_test.exs enumerates cmd -- FILE for every Registry command that takes a file operand, with an explicit skip-and-reason for the rest, so a new command cannot join unclassified.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Testing

• Added new tests
• All existing tests pass

Local gates:

mix compile --warnings-as-errors
mix format --check-formatted
mix credo --strict   # only the pre-existing apply/2 test fixture
mix test
# Finished in 33.4 seconds
# 2 doctests, 62 properties, 5420 tests, 0 failures (5 excluded)
mix docs --warnings-as-errors
mix dialyzer
# Total errors: 13, Skipped: 13, Unnecessary Skips: 0

Checklist

• My code follows the style guidelines of this project
• I have run mix format
• I have run mix credo and addressed any issues
• I have added tests that prove my fix is effective
• New and existing tests pass locally with my changes
• I have updated the CHANGELOG.md