fix(server): broken shutdown_tx in run_server after #1704 merged onto post-#1945 main

[st-gr]

Symptom

cargo build / cargo check -p openshell-server fails on current main (commit 4ee27d99) with:

```
error[E0425]: cannot find value shutdown_tx in this scope
--> crates/openshell-server/src/lib.rs:460:13
|
460 | let _ = shutdown_tx.send(true);
| ^^^^^^^^^^^ help: a local variable with a similar name exists: _shutdown_tx
```

rust-toolchain.toml pins 1.95.0; reproduces under any clean checkout of main.

Cause

crates/openshell-server/src/lib.rs::run_server declares (_shutdown_tx, shutdown_rx) at line 347 (with the unused-prefix _) but uses shutdown_tx.send(true) at line 460. The names don't match — won't compile.

Land-order regression between two individually-correct PRs:

• fix(server): share gateway shutdown channel #1945 fix(server): share gateway shutdown channel (merged 2026-06-17, commit 4c75b854) made the first shutdown_tx in run_server used by adding the shutdown_tx.send(true) call at the new shutdown-signal block. That's correct against the pre-fix(server): share gateway shutdown channel #1945 main where the variable was unused.
• feat(sandbox,providers): add aws-bedrock as a recognized inference provider #1704 feat(sandbox,providers): add aws-bedrock as a recognized inference provider (merged 2026-06-23, commit 4ee27d99) included a drive-by style(server): silence unused-variable lint on first shutdown_tx commit that prefixed the declaration with _. That was correct against the main feat(sandbox,providers): add aws-bedrock as a recognized inference provider #1704 branched from (pre-fix(server): share gateway shutdown channel #1945) where the variable was genuinely unused, but became wrong after fix(server): share gateway shutdown channel #1945 made it used.

Neither PR's CI re-validated the combined post-merge state, so the regression landed on main without surfacing.

This is exactly the integration gap @elezar described in #1946.

Fix

One line in crates/openshell-server/src/lib.rs:

```diff

• let (_shutdown_tx, shutdown_rx) = watch::channel(false);

• let (shutdown_tx, shutdown_rx) = watch::channel(false);
  ```

I verified this locally — applying the rename clears the build error, the rest of the workspace builds clean, and the existing cargo test -p openshell-server --lib suite passes (833/833).

Related

• Introduced by feat(sandbox,providers): add aws-bedrock as a recognized inference provider #1704 (interaction with fix(server): share gateway shutdown channel #1945's earlier change).
• Long-term prevention is what feat(ci): introduce merge queue for main #1946 proposes (merge queue for main).

Happy to open a one-line PR if it speeds things up.