Never overwrite a file the project already maintains

Running pr against a real repository proposed replacing its hand-written
.env.example with a generated stub. The existing file explained that VITE_*
values are inlined into the public bundle and are identifiers rather than
secrets, warned against putting an Appwrite API key there, and carried a working
default. The generated template had none of that and added a spurious DEV entry.

A generated template is a floor for a project that has none, not an improvement
on one a person wrote. pr now skips any artifact whose path already exists,
warns which files it left alone, and fails rather than opening an empty pull
request when every artifact already exists.

Found by running the tool against Barter, not by any test.

Author: bananatruck

crates/launchguard-cli/src/main.rs

@@ -830,9 +830,32 @@ async fn pull_request(options: PullRequestOptions<'_>) -> Result<()> {
     let intent = IntentGenerator
         .generate(&profile, options.provider, None)
         .context("failed to capture deployment intent")?;
-    let files =
+    let generated =
         generate_configuration(&intent).context("failed to generate deployment configuration")?;
 
+    // A file the project already maintains is left alone. A generated template
+    // is a floor for a project that has none, never an improvement on one a
+    // person wrote, and silently replacing hand-written guidance with a
+    // generic stub is a regression no reviewer asked for.
+    let mut files = Vec::new();
+    let mut preserved = Vec::new();
+    for file in generated {
+        if repository.root().join(&file.path).exists() {
+            preserved.push(file.path.clone());
+        } else {
+            files.push(file);
+        }
+    }
+    for path in &preserved {
+        warn!(path = path.as_str(), "keeping the existing file unchanged");
+    }
+    if files.is_empty() {
+        return Err(anyhow!(
+            "every generated file already exists in the repository ({}); nothing to publish",
+            preserved.join(", ")
+        ));
+    }
+
     let capability = CapabilityProbe::default().detect().await.ok();
     let mut decision = PublicationGate
         .evaluate(