Port the Python-authored docs to Ruby

[cassieemb]

Summary

Three core docs the README links are written for the Python SDK — .py example files, Python signatures (self., def …(self)), and Python-only concepts (e.g. FastAPI) — so they don't match the Ruby SDK. A Ruby developer following them gets Python commands and APIs.

Where

• docs/cli_guide.md — entirely Python; commands don't match the Ruby swaig-test.
• docs/api_reference.md — substantial Python signatures; doesn't reflect the Ruby public surface.
• docs/third_party_skills.md — the "write your own skill" guide is Python.

Directive

These are independent files — fine to split across commits/PRs or hand each sub-item to a different agent.

1. docs/cli_guide.md — rewrite against the Ruby swaig-test: real flags, .rb files, the actual --file/--url behavior (coordinate with T5/T6). Run every command shown.
2. docs/api_reference.md — convert signatures/examples to Ruby; reconcile names against the actual public methods (grep the classes under lib/signalwire/).
3. docs/third_party_skills.md — rewrite as a Ruby SkillBase walkthrough (setup, register_tools, params) with a complete runnable example.

Guardrails

Document only what the Ruby code actually provides: flags/modes that bin/swaig-test accepts, and methods present in the public classes under lib/signalwire/. Don't carry over Python-only flags or APIs.

Acceptance

• no .py/Python-framework references remain in the three docs
• every command in cli_guide.md runs as written
• api_reference.md examples are valid Ruby and documented methods exist with the documented arity
• following third_party_skills.md verbatim produces a working custom skill in Ruby

Verify

Execute each documented CLI command; ruby -c any extractable snippets; build the skill guide's example and load it into an agent.