Add the project skeleton and the approved design

Cargo.toml with the release profile and lints, the toolchain pinned to
Rust 1.90.0, a build script that records the git commit for
`cumulus --version`, and rustfmt settings and cargo aliases (`cargo lint`,
`cargo linux`) as in skyshift. DESIGN.md holds the design approved after
the question rounds; README.md has build instructions and the planned keys.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.md

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+# cumulus
+
+Brainstorm in the terminal: ideas as nodes on a 2D canvas, connected, folded
+and color-coded, across as many documents as you like. Vim keys first; the
+mouse works too. Runs on Windows and Linux.
+
+> **Status:** being built, one stage at a time. See [DESIGN.md](DESIGN.md) for
+> the plan; this README grows with the app.
+
+## Build and run
+
+You need Rust ([rustup.rs](https://rustup.rs)). The exact version is pinned in
+`rust-toolchain.toml`, and rustup installs it by itself the first time.
+
+```sh
+cargo build --release
+```
+
+The program is `target/release/cumulus` (`cumulus.exe` on Windows): copy it to
+a folder on your `PATH`.
+
+- **Windows**: nothing else to install. The `.exe` carries its own C runtime
+  and runs on any Windows 10 or 11.
+- **Linux** (Void, Fedora…): build the same way. You can also build a static
+  Linux binary from Windows with `cargo linux`: it lands in
+  `target/x86_64-unknown-linux-musl/release/cumulus` and runs on any
+  distribution.
+
+## Keys
+
+The plan for the everyday keys (all of them are in DESIGN.md):
+
+| Keys              | What they do                                       |
+|-------------------|----------------------------------------------------|
+| `hjkl`            | move to the nearest node that way                  |
+| `gh gj gk gl`     | parent, next sibling, previous sibling, first child |
+| `o` `O` `a`       | new sibling below / above, new child               |
+| `i` or Enter      | edit the title                                     |
+| `dd` `x`          | delete the branch / just the node                  |
+| `u` `Ctrl-r`      | undo / redo                                        |
+| `za` `zf` `zi`    | fold, auto-fold, focus                             |
+| `+` `-`           | zoom in / out                                      |
+| `C` `M` `v`       | connect, move, select                              |
+| `<Space>`         | the action menu                                    |
+| `:` `?`           | command line, help                                 |
+
+After the first key of a sequence (`g`, `z`, `d`, `<Space>`…), a popup lists
+what can follow.
+
+## Development
+
+```sh
+cargo test    # run the tests
+cargo lint    # clippy, treating warnings as errors
+cargo fmt     # format the code
+```