Autocomplete cm6

Editor assessment

'Cm6' is project shorthand for CodeMirror 6, and anyone outside the team will hit a wall: the header communicates nothing about what the autocomplete does, where it appears, or whether it is user-facing. Expanding to `feat(editor): add autocomplete for CodeMirror 6` would cost a few characters and remove the ambiguity, while giving the feature a proper scope. The message does pass the mechanical rules — feat type, lowercase, no trailing period, well under 72 characters, no dangling body. For a feature this visible, though, a body is expected: describing the trigger behavior, shortcuts, or configuration would give release notes something to quote. As shipped, the changelog will record 'Features: autocomplete cm6', which is obscure. Right structure, under-explained feature.