[FEATURE]: Support inline skill invocation using $skill-name

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Support inline skill invocation using $skill-name syntax anywhere in a prompt (not just at the start).

Currently, OpenCode requires skills to be invoked via slash commands at the beginning of a prompt (e.g., /skill-name do something). OpenAI Codex supports mentioning skills inline with $skill-name anywhere in the prompt, allowing more natural phrasing like:
Use $brainstorming to explore ideas for the API design

Reference:
https://developers.openai.com/codex/skills/