fix: case-insensitive trailer dedup and always add Ai-assisted (#25)

## Summary
- Fix case-sensitive `Co-authored-by:` trailer check that missed
variants like `Co-Authored-By:`
- When the co-author email is already present, still add `Ai-assisted:
true` if missing
- Add test for case-insensitive trailer matching and assertions for
`Ai-assisted` in existing dedup tests

## Test plan
- [x] `cargo test` — all 21 tests pass
- [x] `cargo clippy -- -D warnings` — clean

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: alecthomas

src/git.rs

@@ -23,36 +23,50 @@ pub fn append_trailers(commit_msg_file: &PathBuf, agent: &Agent, debug: bool) ->
     let content = fs::read_to_string(commit_msg_file)?;
 
     let addr = Agent::extract_email_addr(agent.email);
-    if content.contains("Co-authored-by:") && content.contains(addr) {
+    let content_lower = content.to_lowercase();
+    let has_co_author = content_lower.contains("co-authored-by:") && content_lower.contains(&addr.to_lowercase());
+    let has_ai_assisted = content_lower.contains("ai-assisted: true");
+
+    if has_co_author && has_ai_assisted {
         if debug {
             eprintln!("\n=== Git Command ===");
             eprintln!("Trailers already present, skipping git interpret-trailers");
         }
         return Ok(());
     }
 
-    let co_authored = format!("Co-authored-by: {}", agent.email);
+    let mut cmd = std::process::Command::new("git");
+    cmd.arg("interpret-trailers").arg("--in-place");
 
-    if debug {
+    if !has_co_author {
+        let co_authored = format!("Co-authored-by: {}", agent.email);
+        if debug {
+            eprintln!("\n=== Git Command ===");
+            eprintln!(
+                "git interpret-trailers --in-place --trailer \"{}\" --if-exists addIfDifferent --trailer \"Ai-assisted: true\" \"{}\"",
+                co_authored,
+                commit_msg_file.display()
+            );
+        }
+        cmd.arg("--trailer")
+            .arg(&co_authored)
+            .arg("--if-exists")
+            .arg("addIfDifferent");
+    } else if debug {
         eprintln!("\n=== Git Command ===");
         eprintln!(
-            "git interpret-trailers --in-place --trailer \"{}\" --if-exists addIfDifferent --trailer \"Ai-assisted: true\" \"{}\"",
-            co_authored,
+            "git interpret-trailers --in-place --trailer \"Ai-assisted: true\" \"{}\"",
             commit_msg_file.display()
         );
     }
 
-    let output = std::process::Command::new("git")
-        .arg("interpret-trailers")
-        .arg("--in-place")
-        .arg("--trailer")
-        .arg(&co_authored)
-        .arg("--if-exists")
-        .arg("addIfDifferent")
-        .arg("--trailer")
-        .arg("Ai-assisted: true")
-        .arg(commit_msg_file)
-        .output()?;
+    if !has_ai_assisted {
+        cmd.arg("--trailer").arg("Ai-assisted: true");
+    }
+
+    cmd.arg(commit_msg_file);
+
+    let output = cmd.output()?;
 
     if !output.status.success() {
         return Err(std::io::Error::other(format!(

src/main.rs

@@ -258,6 +258,34 @@ mod tests {
         let agent = Agent::find_by_name("claude").unwrap();
         append_trailers(&file.path().to_path_buf(), agent, false).unwrap();
 
+        let content = fs::read_to_string(file.path()).unwrap();
+        let co_author_count = content.matches("noreply@anthropic.com").count();
+        assert_eq!(
+            co_author_count, 1,
+            "Should not add duplicate trailer for same email address, found {} occurrences",
+            co_author_count
+        );
+        assert!(
+            content.contains("Ai-assisted: true"),
+            "Should still add Ai-assisted trailer even when co-authored-by already exists"
+        );
+    }
+
+    #[test]
+    fn test_append_trailers_skips_existing_email_different_case() {
+        // The "Co-Authored-By" key can have varying capitalisation
+        let mut file = NamedTempFile::new().unwrap();
+        writeln!(file, "Initial commit").unwrap();
+        writeln!(file).unwrap();
+        writeln!(
+            file,
+            "Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>"
+        )
+        .unwrap();
+
+        let agent = Agent::find_by_name("claude").unwrap();
+        append_trailers(&file.path().to_path_buf(), agent, false).unwrap();
+
         let content = fs::read_to_string(file.path()).unwrap();
         // Should NOT have added a second Co-authored-by for noreply@anthropic.com
         let co_author_count = content.matches("noreply@anthropic.com").count();
@@ -266,6 +294,11 @@ mod tests {
             "Should not add duplicate trailer for same email address, found {} occurrences",
             co_author_count
         );
+        // But SHOULD have added Ai-assisted
+        assert!(
+            content.contains("Ai-assisted: true"),
+            "Should still add Ai-assisted trailer even when co-authored-by already exists"
+        );
     }
 
     #[test]