Rename the CLI package to tsunagi and its executable to tsng

The command line package is now `tsunagi` and builds the `tsng` executable,
which the packages, the unit, the installers and the workflows install and
run. The core library package becomes `tsunagi-core`, keeping the library name
`tsunagi`, so no code changes `use tsunagi::...`. Directories follow
(crates/tsunagi-core, crates/tsunagi). Every command shown to a person is now
`tsng ...`.

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

docs/sync-model.md

@@ -1,6 +1,6 @@
 # Planned state synchronisation
 
-**The first slice of this model is now implemented**, in `crates/tsunagi/src/state/`, and is
+**The first slice of this model is now implemented**, in `crates/tsunagi-core/src/state/`, and is
 used for one thing: IPv4 overlay addresses. What follows describes the whole
 model; the section at the end says exactly which parts exist.
 
@@ -77,7 +77,7 @@ migrations for this.
 
 ## What exists today
 
-Implemented, in `crates/tsunagi/src/state/`:
+Implemented, in `crates/tsunagi-core/src/state/`:
 
 * signed records, one per author per network, each holding that author's
   complete current statement rather than a delta;