Split the system level and the command line into a workspace

First step of separating the layers. The library and the binary are now
crates/tsunagi and crates/tsunagi-cli, which means the plugin crate to
come can be told apart from the core by the compiler rather than by
discipline.

Falls out of it immediately: the CLI's dependencies stop being features
of the library. clap, anstream and tracing-subscriber were optional
dependencies behind a `cli` feature that every library user had to
remember to turn off; now they belong to the crate that uses them, and
the library defaults to no features at all.

The one test that drives the binary moved beside it — a library cannot
depend on a binary built from a crate that depends on the library — and
was rewritten against the public API instead of the test harness.

AGENTS.md said to prefer one crate. It now says the system level and its
plugins are separate crates, for the reason above, and that everything
else stays one crate.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -3821,12 +3821,9 @@ checksum = "e421abadd41a4225275504ea4d6566923418b7f05506fbc9c0fe86ba7396114b"
 name = "tsunagi"
 version = "0.1.0"
 dependencies = [
- "anstream",
- "anstyle",
  "boringtun",
  "bytes",
  "caps",
- "clap",
  "data-encoding",
  "directories",
  "fs4",
@@ -3856,6 +3853,23 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "tsunagi-cli"
+version = "0.1.0"
+dependencies = [
+ "anstream",
+ "anstyle",
+ "clap",
+ "iroh",
+ "netwatch",
+ "simple-dns",
+ "tempfile",
+ "tokio",
+ "tracing",
+ "tracing-subscriber",
+ "tsunagi",
+]
+
 [[package]]
 name = "tun"
 version = "0.8.14"