docs: refresh shell AST skill

[aliou]

Summary

Bring skills/sh-ast/SKILL.md in line with what the parser actually does.

• Document SelectClause, CoprocClause, TimeClause, BraceExp, and ExtGlob.
• Add a positions section (pos/end, Pos, NO_POS).
• Document recoverErrors, parseStmtsSeq, parseWordsSeq, and splitBraces, with a brace-expansion example.
• Replace stale limitations (positions, extglobs, arithmetic strings) with the ones that still hold.
• Patch changeset for the bundled skill.

Replaces #5, which was written on a remote branch.

Verification

Every code snippet and node-type claim in the skill was exercised with a throwaway vitest file (brace example produces a BraceExp with 3 elements, seq helpers yield 3 statements / 3 words, recoverErrors returns errors, select/time/coproc parse to their nodes). That file was removed before committing.

pnpm test (20 files, 261 tests) and pnpm typecheck pass.