[FEATURE]: autocomplete skill invocations in middle of prompt

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Skill invocation and tab completion currently work only when the skill command appears at the beginning of the prompt. Support detecting and autocompleting skill invocations anywhere in prompt text, similar to Claude Code.

For example, when typing /i-ha in the middle of a prompt, autocomplete should show only the matching skill and complete directly to /i-have-adhd, without presenting selectable options.

This would allow users to add skills alongside normal prompt text instead of requiring skill commands to appear only at the beginning.