Conformance gate for docs/skills/examples; remove remaining dead grammar (1.4, 2.3)

[ako]

Plan items 1.4 and 2.3 in the proposal.

• Conformance gate: parse everything in mxcli syntax, the skills, docs-site and mdl-examples with --deprecations=error, with an allowlist that may only shrink.
• Dead grammar: remove the grammar that always fails, such as grant … on workflow, case … else and text/statictext/legacydatagrid, if any survived Silent drops: throw, float/currency/date types, parenthesised association options, enum-value docs and index names #706.

Tracking: #714. Language rules: ADR-0010/0011/0012. Changes of meaning apply only under the mdl 1 header; old spellings stay as registered deprecated aliases, each with an fmt --upgrade rewrite (#742).