feat: ignore backup files in .gitignore by default

This change ensures that backup files created by AgentSync (e.g., when
replacing an existing file with a symlink or during skill updates) are
automatically ignored by Git.

Changes:
- In `src/config.rs`, `all_gitignore_entries()` now includes:
  - `{destination}.bak.*` for every target destination.
  - `.agents/skills/*.bak` for skill update backups.
- Added a unit test `test_all_gitignore_entries_includes_backup_patterns` to
  verify the new behavior.
- Verified that all 191 tests pass.

Co-authored-by: yacosta738 <33158051+yacosta738@users.noreply.github.com>

Author: google-labs-jules[bot]

src/config.rs

@@ -279,12 +279,14 @@ impl Config {
     pub fn all_gitignore_entries(&self) -> Vec<String> {
         let mut entries: BTreeSet<String> = self.gitignore.entries.iter().cloned().collect();
 
+        entries.insert(".agents/skills/*.bak".to_string());
         // Add destinations from all enabled agents and their known patterns
         for (agent_name, agent) in &self.agents {
             if agent.enabled {
                 // Add target destinations
                 for target in agent.targets.values() {
                     entries.insert(target.destination.clone());
+                    entries.insert(format!("{}.bak.*", target.destination));
                 }
 
                 // Add known ignore patterns for this agent
@@ -1262,4 +1264,22 @@ mod tests {
         assert!(config.gitignore.enabled);
         assert_eq!(config.agents["copilot"].description, "GitHub Copilot");
     }
+    #[test]
+    fn test_all_gitignore_entries_includes_backup_patterns() {
+        let toml = r#"
+            [agents.test]
+            enabled = true
+            [agents.test.targets.main]
+            source = "README.md"
+            destination = "OUTPUT.md"
+            type = "symlink"
+        "#;
+
+        let config: Config = toml::from_str(toml).unwrap();
+        let entries = config.all_gitignore_entries();
+
+        assert!(entries.contains(&"OUTPUT.md".to_string()));
+        assert!(entries.contains(&"OUTPUT.md.bak.*".to_string()));
+        assert!(entries.contains(&".agents/skills/*.bak".to_string()));
+    }
 }