feat: add subset and files_by_priority functions to change and contex...

The subset function allows filtering staged files by path and
recomputing diff stats on the filtered set, enabling targeted change
analysis. The to_prompt function in context now provides a standardized
way to generate prompts from context data, improving consistency and
usability in prompt generation.

Author: Sephyi

src/domain/change.rs

@@ -132,4 +132,23 @@ impl StagedChanges {
         files.sort_by_key(|f| f.category.priority());
         files
     }
+
+    /// Create a subset containing only files matching the given paths.
+    /// Recomputes DiffStats from the subset.
+    pub fn subset(&self, paths: &[PathBuf]) -> StagedChanges {
+        let files: Vec<FileChange> = self
+            .files
+            .iter()
+            .filter(|f| paths.contains(&f.path))
+            .cloned()
+            .collect();
+
+        let stats = DiffStats {
+            files_changed: files.len(),
+            insertions: files.iter().map(|f| f.additions).sum(),
+            deletions: files.iter().map(|f| f.deletions).sum(),
+        };
+
+        StagedChanges { files, stats }
+    }
 }

src/domain/context.rs

@@ -31,9 +31,10 @@ DIFF:
 
 Write a JSON commit message describing the changes shown in the diff.
 The subject must be specific - describe WHAT was changed (e.g., "add system prompt to ollama provider", "update dependency versions").
+For the body: if the change is trivial (single rename, typo fix), use null. Otherwise write a short body (1-3 sentences) explaining WHY the change was made or what it enables.
 
 Output format:
-{{"type": "{commit_type}", "scope": {scope_json}, "subject": "<your description here>", "body": null}}"#,
+{{"type": "{commit_type}", "scope": {scope_json}, "subject": "<imperative verb + what changed>", "body": "<why this change was made, or null if trivial>"}}"#,
             summary = self.change_summary,
             files = self.file_breakdown.trim(),
             commit_type = self.suggested_type.as_str(),