fix(livekit): replace llm_cost_usd with token counters

[adityajha2005]

Summary

otel-livekit-style was the only bundled skill still instructing developers to track llm.cost_usd as an app-side metric. Every other skill (fastapi, python, nextjs, onboarding) explicitly prohibits this because Superlog computes cost centrally from provider/model/token data.

This PR brings otel-livekit-style/SKILL.md in sync with the rest of the skill library. It is a direct port of the same fix already merged in the skills repo: superloglabs/skills#12.

Changes

• Frontmatter description - updated from "LLM turn cost metrics" to "LLM turn token counters" to match the actual guidance
• LLM Turns bullet list - removed cost counter; inlined the metric names (llm.tokens.input, llm.tokens.output) directly into the token counters bullet so they're unambiguous
• Prohibition note added - "Do not add llm.cost_usd metrics; Superlog estimates cost centrally from provider/model/token data." - exact wording matches otel-python-style and otel-fastapi-style
• Code example replaced - llm_cost_usd.add(cost, {...}) → shared attrs dict + llm_tokens_input.add(input_tokens, attrs) +llm_tokens_output.add(output_tokens, attrs)

Relation to skills repo

The identical fix was merged upstream in superloglabs/skills#12.
This PR ports it to the bundled copy of the skill shipped inside the CLI so both stay in sync.

[adityajha2005]

hey @arseniycodes do let me know if any more changes required in this pr