Commits of agents carry a co-author trailer naming the model

A git wrapper (docker/git-coauthor -> /usr/local/lib/codecraft/bin/git) adds
--trailer "$CODECRAFT_COMMIT_TRAILER" right after the commit subcommand; the
agent shell puts it first on PATH and sets the trailer in the script itself
(works inside the sandbox, which clears the environment). Default text:
"Co-Authored-By: {model} ({provider}) via CodeCraft" (server.commit_trailer,
Settings -> General; empty = off). Git skips duplicates (amend, model wrote it).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/config.rs

@@ -307,8 +307,13 @@ pub struct ServerConfig {
     /// Bearer token for GET /metrics (Prometheus); empty = /metrics disabled.
     #[serde(skip_serializing_if = "String::is_empty")]
     pub metrics_token: String,
+    /// Trailer added to every git commit an agent makes ({model}, {provider}, {model_id});
+    /// empty = off.
+    pub commit_trailer: String,
 }
 
+pub const DEFAULT_COMMIT_TRAILER: &str = "Co-Authored-By: {model} ({provider}) via CodeCraft";
+
 impl Default for ServerConfig {
     fn default() -> Self {
         Self {
@@ -327,6 +332,7 @@ impl Default for ServerConfig {
             max_upload_mb: 500,
             update_url: "https://git.familie-zink.org/api/v1/repos/zink/codecraft/releases/latest".into(),
             metrics_token: String::new(),
+            commit_trailer: DEFAULT_COMMIT_TRAILER.into(),
         }
     }
 }