fix(cli): stop watch taking an option's value as the watched path

[Chemaclass]

🤔 Background

Related #1291

cmd_watch knew only -f/--filter as consuming a value, so every other option's value fell through to the positional branch and became the path being polled. bashunit watch --tag slow tests/ polled a directory named slow and handed the real path to --tag. That case is loud only because of the path validation added in #1263; whenever a value happens to name a real directory, watch polls the wrong one and drops the option silently.

💡 Changes

• Options may now appear in any position; only a true positional becomes the path
• The set of value-taking options lives in one shared predicate instead of a second hardcoded list that can drift out of step with cmd_test
• An anti-drift test derives the expected set from cmd_test's own parser — an option takes a value there precisely when its case arm ends in an unconditional shift — and checks both directions, so neither a new option nor a removed one can slip through. Mutation-tested: dropping --tag and adding a bogus entry each fail it by name
• Docs drop the "put the path first" caveat