docs: refresh shell AST skill

[aliou]

Summary

• document current command and word-part AST node types
• add source positions, recoverable parsing, lazy sequence parsing, and brace splitting
• replace outdated limitations around positions, extglobs, and arithmetic ASTs
• add a patch changeset for the bundled skill update

Validation

Using Node 22.23.1 and pnpm 10.26.1:

• pnpm install --frozen-lockfile
• pnpm check
• pnpm typecheck
• pnpm test (19 files, 241 tests)
• pnpm build
• pnpm changeset status
• git diff --check
• static skill frontmatter, size, and stale-claim assertions