chore(maestro): retarget installer, e2e harness, and docs at unified maestro binary (NAP-245)

- Installer: one `cargo install maestro` instead of two; ExecStart becomes
  `~/.cargo/bin/maestro daemon`; user unit renamed `maestrod.service` →
  `maestro.service`; configure step is `maestro daemon configure`.
- E2E harness: both `Daemon` and `Registry` subprocess wrappers shell out to
  the unified `maestro` binary with the appropriate subcommand.
- State dir: `~/.local/state/maestrod` → `~/.local/state/maestro`.
- Docs: CLAUDE.md repo layout, design spec repo layout + "Daemon (`maestro daemon`)"
  heading, INSTALL.md (full rewrite of CLI references + new thin-builds section).

crates/maestro-e2e/src/daemon.rs

@@ -189,8 +189,9 @@ fn launch(
     tmux_tmpdir: &std::path::Path,
     opts: &DaemonOptions,
 ) -> Child {
-    let bin = workspace_target_bin("maestro-daemon", "maestrod");
+    let bin = workspace_target_bin("maestro", "maestro");
     let mut cmd = Command::new(&bin);
+    cmd.arg("daemon");
     cmd.env_clear()
         .env(
             "PATH",

crates/maestro-e2e/src/registry.rs

@@ -27,8 +27,9 @@ impl Registry {
         let addr = SocketAddr::from((Ipv4Addr::LOCALHOST, port));
         let home = TempDir::new().expect("home tmpdir");
 
-        let bin = workspace_target_bin("maestro-registry", "maestro-registry");
+        let bin = workspace_target_bin("maestro", "maestro");
         let child = Command::new(&bin)
+            .arg("registry")
             .env_clear()
             // PATH so tmux etc. resolve. We need a few essentials.
             .env(