Remove all alternate update paths — fleet redeploy is release + upgrade all

Killed:
- rebuild_and_restart (in-container cargo self-compile): the MCP tool,
  the `rebuild` chat command in all four transports, the web-ui bot
  command, and the underlying function. This was the path that caused
  the exec() deadlocks.
- upgrade_sled gateway MCP tool: second entry point to sled upgrades,
  defaulted to serving the gateway's own macOS binary to Linux sleds.
- GET /api/huskies-binary (both sled and gateway route trees): served
  current_exe(), wrong platform when the gateway is macOS. Superseded
  by /api/artifacts/ which now also serves on the gateway route tree.
- `huskies upgrade` CLI subcommand and --source flag: third way of
  doing the same download-and-replace. Escape hatch for a bricked sled
  is `docker cp` + restart.

Kept, distinct jobs: `project-rebuild` (container/image updates),
`rebuild gateway` + script/local-release (gateway self-update).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019fHdm92yjvguPi2LiXfLB9

server/src/gateway/mod.rs

@@ -65,10 +65,11 @@ pub fn build_gateway_route(state_arc: Arc<GatewayState>) -> impl poem::Endpoint
             "/gateway/agents/:id/assign",
             poem::post(gateway_assign_agent_handler),
         )
-        // Binary self-update: serve the gateway binary so sleds can download it.
+        // Artifact store: sleds download published binaries from here during
+        // `upgrade all` — never from anywhere else.
         .at(
-            "/api/huskies-binary",
-            poem::get(crate::http::serve_binary_handler),
+            "/api/artifacts/:filename",
+            poem::get(crate::http::serve_artifact_handler),
         )
         .data(state_arc)
 }