core: call the word after as an alias, in messages, the spec and the docs

[jbpros]

Second step of #244. ergo called the word that stands for something longer a "name", the same word it uses for requirements, checks and subjects. It's now an alias everywhere a reader meets it. That covers the word after as on a for line, the old syntax's each_as and as names and the $name that refers to one, and a bare word after named by. REFERENCE.md defines it once: "An alias is a short word that stands for something longer: as pr makes pr an alias for each pull request in turn."

Before	After
name given twice	alias given twice
invalid name	invalid alias
unknown name <word>	unknown alias <word>
<word> can't be a name	<word> can't be an alias
<name given twice>, <invalid name> in expressions	<alias given twice>, <invalid alias>

What stays "name":

• A sentence path starting with $ that isn't $params or $input still says unknown name $foo: in a sentence, an alias is a bare word, so $foo isn't one. A test says so.
• <unknown name $input> and <unknown name $params> in old-syntax expressions stay too, since those aren't aliases.

The old syntax's $artifact, a chained subject's parent, now says unknown alias too, since a chain's parent is an alias by #244's decisions.

The ABNF rule name is now alias, and the rule id sentence.for.name is now sentence.for.alias, with its 17 cites updated. Internal Rego rule names are unchanged.

Changes the report: the messages above, in $well_formed inputs and in expressions.

Checks

• Every check in AGENTS.md passes: 1258 tests, no uncovered lines, 1110 spec cases. spec/check_grammar.py finds 0 disagreements.
• The JS Wasm runtime passes within its budgets.
• On every spec case from main and every corpus line on two inputs (1740 reports), main and this branch differ in 6 reports, and only by these messages.

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