feat: deduplicate trailers by email address (#9)

Checks existing commit message trailers by extracted email address
instead of the full "Name <email>" string. Prevents adding a duplicate
Co-authored-by when the agent already wrote one with a different display
name (e.g. "Claude Opus 4.6" vs "Claude Code" both using
noreply@anthropic.com).

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: baldawarishi

src/git.rs

@@ -22,7 +22,8 @@ pub fn find_git_root(start_path: &Path) -> Option<PathBuf> {
 pub fn append_trailers(commit_msg_file: &PathBuf, agent: &Agent, debug: bool) -> std::io::Result<()> {
     let content = fs::read_to_string(commit_msg_file)?;
 
-    if content.contains("Co-authored-by:") && content.contains(agent.email) {
+    let addr = Agent::extract_email_addr(agent.email);
+    if content.contains("Co-authored-by:") && content.contains(addr) {
         if debug {
             eprintln!("\n=== Git Command ===");
             eprintln!("Trailers already present, skipping git interpret-trailers");

src/main.rs

@@ -242,15 +242,36 @@ fn main() {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use agent::KNOWN_AGENTS;
     use std::fs;
     use std::io::Write;
     use tempfile::NamedTempFile;
 
+    #[test]
+    fn test_append_trailers_skips_existing_email_different_name() {
+        // Simulate Claude Code already having added a trailer with a different display name
+        // but the same email address (e.g. "Claude Opus 4.6 <noreply@anthropic.com>")
+        let mut file = NamedTempFile::new().unwrap();
+        writeln!(file, "Initial commit").unwrap();
+        writeln!(file).unwrap();
+        writeln!(file, "Co-authored-by: Claude Opus 4.6 <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();
+        assert_eq!(
+            co_author_count, 1,
+            "Should not add duplicate trailer for same email address, found {} occurrences",
+            co_author_count
+        );
+    }
+
     #[test]
     fn test_dedup_agents_removes_duplicates() {
-        let claude = &KNOWN_AGENTS[0]; // Claude Code
-        let amp = &KNOWN_AGENTS[8]; // Amp
+        let claude = Agent::find_by_name("claude").unwrap();
+        let amp = Agent::find_by_name("amp").unwrap();
         let agents = vec![claude, amp, claude];
         let deduped = dedup_agents(agents);
         assert_eq!(deduped.len(), 2);
@@ -316,7 +337,7 @@ mod tests {
         let mut file = NamedTempFile::new().unwrap();
         writeln!(file, "Initial commit").unwrap();
 
-        let agent = &KNOWN_AGENTS[0];
+        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();
@@ -329,7 +350,7 @@ mod tests {
         let mut file = NamedTempFile::new().unwrap();
         writeln!(file, "Initial commit").unwrap();
 
-        let agent = &KNOWN_AGENTS[0];
+        let agent = Agent::find_by_name("claude").unwrap();
         append_trailers(&file.path().to_path_buf(), agent, false).unwrap();
         let content1 = fs::read_to_string(file.path()).unwrap();
 
@@ -363,8 +384,8 @@ mod tests {
         let mut file = NamedTempFile::new().unwrap();
         writeln!(file, "Initial commit").unwrap();
 
-        let agent1 = &KNOWN_AGENTS[0]; // Claude
-        let agent2 = &KNOWN_AGENTS[8]; // Amp
+        let agent1 = Agent::find_by_name("claude").unwrap();
+        let agent2 = Agent::find_by_name("amp").unwrap();
 
         append_trailers(&file.path().to_path_buf(), agent1, false).unwrap();
         append_trailers(&file.path().to_path_buf(), agent2, false).unwrap();