TOML script targets removed, script fields aligned with annotations

Once 0.13.0 removes TOML-backed script target parsing, so fabrik.toml is reserved for workspace configuration. Script annotations across parsers, shellspecs, docs, and examples now use the capitalised # Once marker instead of # once, and inline script entries share the same public contract field names as annotated script files (input, output, env, cwd). Existing [[task]] script entries must be migrated to [[target]] with kind = "script" and [target.script] before upgrading.