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>

LICENSE

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+MIT License
+
+Copyright (c) 2026 bryant
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.