Replace openrouter-rs with in-tree client and require sandbox

Remove the openrouter-rs dependency in favor of a minimal in-tree
OpenRouter chat-completions client, and drop the BOT_NAME and
SANDBOX_ENABLED config options. Reviews now always run inside the
sandbox, and the review prompt asks the model to read files with the
available tools instead of embedding the diff.

crates/herald-server/src/consts.rs

@@ -27,25 +27,29 @@ pub const REVIEW_PROMPT: &str = "
 
     This is the user comment: \"{comment}\"
 
-    The code changes (only added lines, with line numbers):
+    The pull request changes these files and lines:
 
-    {diff}
+    {changes}
 
-    Please review the code changes and provide feedback.
+    `added` line numbers refer to the new version of the file, `removed` line
+    numbers to the old version, as they appear in the diff.
 
-    IMPORTANT: the `line` field must be the line number shown before each line.
-    The provided code has the format: `filename:line:code`
+    The code is not provided: read the files you need with the available tools
+    before answering. Review only the listed lines.
 
     Return your feedback, in french, with only this json format, reviews must contain each review
     All fields are mandatory.
-    (filename field must contain the full path with extension) and comment must contain a final summary:
+    (filename field must contain the full path with extension; line must be one of the
+    listed line numbers for that file, and side must be \"added\" when the line comes
+    from the `added` list or \"removed\" when it comes from the `removed` list)
+    and comment must contain a final summary:
 
     {
         \"reviews\": [
             {
                 \"filename\": \"\",
                 \"line\": ,
-                \"code\": \"\",
+                \"side\": \"\",
                 \"message\": \"\"
             }
         ],