chore(x): improve developer CLI with help, structure, and test filters (#32665)

## Summary
- Add per-command `--help`/`-h` with thorough descriptions (useful for
LLMs and AI agents)
- Extract CLI logic from `./x` into `tools/x/main.ts` for
maintainability
- Replace manual ANSI escapes with `@std/fmt/colors`
- Merge duplicate command/registry structures into a single
`buildCommands()`
- Test commands (`test-unit`, `test-node`, `test-compat`, `test-spec`)
now require a filter argument and support `--list` to discover available
tests

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: bartlomieju