fix: support symbolic chain operators on ruby 2.6

[rupayon123]

The project declares Ruby 2.6 support, but piping command objects fails there before reaching the shell: method_missing receives a Symbol and calls start_with? on it.

Normalize the method name to a string and use an anchored regex in both Command and Chainer. This preserves the existing operator classification while supporting the declared minimum Ruby version. Add regression coverage for a three-command pipeline and ordinary missing-method fallback.

Validation on Ruby 2.6.10/macOS:

• Before: a real printf | cat | cat pipeline raises NoMethodError: undefined method start_with? for :|:Symbol.
• After: pipeline rendering, actual execution, the bang-method DSL pipeline, and ordinary missing-method fallback all pass in a temporary directory.
• All three edited Ruby files pass syntax checks; git diff --check passes.
• Full RSpec/RuboCop was not run locally: the project's current Bundler/debug tooling is not available in the system Ruby environment. Upstream CI remains necessary.

No command escaping or operator allowlist change is included. Prepared with AI assistance. This is independent of PR #77's debug-failure work.