Added SKILL.md for agents to use this library

[asgvard]

The main ideas behind this skill:

• Doc-linked, not doc-duplicated. Top of the file has a single root URL plus a curated map of which doc page answers which question (concepts, boundaries, key mapping, recipes, etc.).
• Links to the latest repo docs folder, not the website, as it is meant for agents, and it's easier to read docs from repo than from website.
• Mental model first. Explicitly names the two component types/shapes (leaf vs container) and the two non-negotiables: attach ref, wrap container children in FocusContext.Provider. Quite a few bugs in this lib are linked to one of those.
• Patterns-first. Kept the most useful idioms (button, container, modal-with-trap, programmatic focus, conditional focusability) and added a short onArrowPress blocking note since that's a frequent need not obvious from a hook signature alone.
• Params table + return values for useFocusable, plus a top-level API import block — enough to write code without opening docs, with a pointer to docs/api-reference/ when signatures matter.
• Debugging section maps each common symptom to the most likely cause, instead of listing tools generically.
• Frontmatter uses the name + description as a standard for agentic skills.
• Top level note in README.md on how to install the skill via npx skills

No changesets, purely docs update.

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