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>

README.md

@@ -0,0 +1,48 @@
+# invoiceninja-cli
+
+A command-line client for the [Invoice Ninja](https://www.invoiceninja.com/)
+v5 API, built on [`invoiceninja-sdk`](https://crates.io/crates/invoiceninja-sdk).
+
+> **Early scaffold.** Only the `clients` resource is wired up so far, as the
+> template for adding the rest — see `CLAUDE.md` for the pattern.
+> [`GOALS.md`](GOALS.md) covers what the crate is for and what it is not.
+
+## Development
+
+Enter the dev shell (Rust toolchain via `rust-overlay`, matching the SDK's):
+
+```sh
+nix develop
+```
+
+## Usage
+
+Every subcommand needs an API token and, for a self-hosted install, a base
+URL. Both can be passed as flags or environment variables (the same names
+the SDK's own examples use):
+
+```sh
+export IN_API_TOKEN=...
+# export IN_BASE_URL=https://your-domain.example/api/v1   # self-hosted only
+
+invoiceninja-cli clients list
+invoiceninja-cli clients list --all --status active,archived
+invoiceninja-cli clients get <id>
+invoiceninja-cli clients create --data '{"name": "Acme Inc"}'
+invoiceninja-cli clients update <id> --data '{"city": "Springfield"}'
+invoiceninja-cli clients delete <id>
+```
+
+`create` and `update` read the record body as JSON, from `--data` inline,
+`--file <path>` (`-` for stdin), or stdin if neither flag is given.
+`update` fetches the current record first and overlays the given fields on
+top of it, so you only need to mention what's changing — see the comment in
+`src/body.rs` for why that matters (Invoice Ninja's `PUT` is a full replace,
+not a patch).
+
+Add `--output json` to any command for the full record instead of the
+table view's few key columns:
+
+```sh
+invoiceninja-cli --output json clients get <id>
+```