perf: eliminate unnecessary clones and improve API ergonomics

- PromptBuilder::new now takes &[String] instead of Vec<String>
- GeminiClient::new now takes &str, &[String] instead of owned values
- FileBatch::from_path now takes &Path instead of PathBuf
- categorize_files_offline now takes Vec<String> (ownership) instead of &[String]
- handle_offline_organization now takes FileBatch by value

These changes eliminate ~5-50 KB of unnecessary allocations for typical
file counts, reduce allocator pressure, and improve API clarity by properly
expressing ownership semantics.

No functional changes - all tests pass.

src/gemini/prompt.rs

@@ -23,7 +23,7 @@ pub struct PromptBuilder {
 }
 
 impl PromptBuilder {
-    pub fn new(file_list: Vec<String>) -> Self {
+    pub fn new(file_list: &[String]) -> Self {
         Self {
             file_list: file_list.join(", "),
         }