[M3] add cargoxx build --no-build

CHANGELOG.md

@@ -78,6 +78,14 @@ All notable changes to cargoxx will be documented in this file.
   and `[build]` honoring `warnings_as_errors` and `sanitizers`. Source
   paths emitted relative to `build/` (i.e. prefixed with `../`).
   Output is deterministic. `tests/codegen_cmake.cpp` covers 11 cases.
+- `cargoxx build --no-build` end-to-end. Reads `Cargoxx.toml`,
+  discovers the layout, opens the curated linkdb, resolves a `Recipe`
+  per manifest dep, synthesizes a fresh `Cargoxx.lock`, and writes
+  `flake.nix`, `build/CMakeLists.txt`, and `Cargoxx.lock`. With no
+  resolver yet (M5), `nixpkgs_rev` is left null and the generated flake
+  falls back to the `nixos-unstable` branch. Without `--no-build`, the
+  command still generates files but returns a `NotImplemented` error
+  pointing at M4. `tests/cmd_build.cpp` covers 8 cases.
 - SQLite overlay: `Database::open(overlay_path)` now opens (and creates,
   if missing) a per-user `linkdb.sqlite` cache, applying the schema from
   `SPEC.md` §9 idempotently. Default path is