Initial commit: clients resource scaffold over invoiceninja-sdk

A command-line client for the Invoice Ninja v5 API, built on the
invoiceninja-sdk crate (consumed from crates.io, 0.2.0). Only the
clients resource is wired up so far, as the template the rest follow
- see CLAUDE.md and GOALS.md for the reasoning behind the shape of
things (the update fetch-merge-write pattern, why there's no Cargo
workspace with the SDK, the local-checkout override for
co-development, and the Nix flake packaging via packages.default and
overlays.default).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.editorconfig

@@ -0,0 +1,18 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.rs]
+indent_style = space
+indent_size = 4
+
+[*.{yml,yaml,toml,nix}]
+indent_style = space
+indent_size = 2
+
+[*.md]
+trim_trailing_whitespace = false