Discover this machine's providers instead of asserting them

A fresh install wrote a `claude-cli` provider into the local setup
unconditionally. Nothing looked for `claude`; the list was hardcoded in
`Config::seed`, so on any machine without it -- which is every machine but
the dev VM -- the phone was offered a provider that cannot spawn, stated
with exactly the confidence of one that had been checked.

Discovery already existed and was already right: `setups::discover` probes
with `command -v` over the transport, includes echo for the local one
because it runs in-process, and records the resolved path rather than the
bare name. Only the local setup skipped it, which is the one place the
answer felt obvious enough not to ask.

So `seed` now takes the providers it is given, and seeding asks this
machine the same question it asks any other. It moved out of
`SessionManager::new` into an awaited step in main, because asking is I/O
and a constructor that quietly spawns a subprocess surprises every caller.
A discovery that fails seeds `echo` alone and says so, since echo is true
wherever this server runs -- falling back to the hardcoded list would be
the same bug with an extra step.

The test that covered this agreed with the bug, because both were written
from the same assumption: it asserted the seed contains `claude-cli`. It
now asserts the opposite -- that the seed invents nothing -- and the
session tests seed echo explicitly rather than relying on a constructor
that would make them pass or fail on whether `claude` happens to be
installed on whoever runs them.

Verified by running a server on a PATH holding only `sh`: it seeds `echo`
alone. With claude and llama-server present it finds both. 34 tests.

server/src/main.rs

@@ -109,6 +109,11 @@ async fn main() -> Result<()> {
         SessionManager::new(config_path.clone(), data_dir, models_dir.clone())
             .with_context(|| format!("failed to load {}", config_path.display()))?,
     );
+    // After construction rather than inside it: seeding asks this machine
+    // what it has, which is I/O, and a constructor that quietly runs a
+    // subprocess is a surprise to every caller including the tests.
+    manager.seed_setup().await?;
+
     tracing::info!("config: {}", config_path.display());
     tracing::info!("models: {}", models_dir.display());
     for setup in manager.setups() {