feat(context): detect source-to-test file correlations in staged changes

Surfaces source-to-test file relationships in the prompt so the LLM can
reason about whether implementation and test changes are paired.
Correlations are matched by file stem and capped at 5 entries to avoid
noise.

Author: Sephyi

src/domain/context.rs

@@ -39,6 +39,9 @@ pub struct PromptContext {
     /// Import/use statement changes detected from diff.
     /// e.g., "analyzer: added use crate::domain::DiffHunk"
     pub import_changes: Vec<String>,
+    /// Source-to-test file correlations detected from staged changes.
+    /// e.g., "src/services/context.rs <-> tests/context.rs (test file)"
+    pub test_correlations: Vec<String>,
 }
 
 impl PromptContext {
@@ -74,6 +77,19 @@ impl PromptContext {
             )
         };
 
+        let related_section = if self.test_correlations.is_empty() {
+            String::new()
+        } else {
+            format!(
+                "\nRELATED FILES:\n{}\n",
+                self.test_correlations
+                    .iter()
+                    .map(|c| format!("  {}", c))
+                    .collect::<Vec<_>>()
+                    .join("\n")
+            )
+        };
+
         // Calculate available chars for subject after type(scope)[!]: prefix
         let prefix_len = self.suggested_type.as_str().len()
             + self
@@ -139,7 +155,7 @@ impl PromptContext {
 SUMMARY: {summary}
 FILES: {files}
 SUGGESTED TYPE: {commit_type}{scope}
-{group_rationale}{evidence}{primary_change}{symbols}{connections}{imports}
+{group_rationale}{evidence}{primary_change}{symbols}{connections}{imports}{related}
 DIFF:
 {diff}
 {constraints}{breaking}{metadata_breaking}{locale}{focus}{history}
@@ -160,6 +176,7 @@ Respond with ONLY this JSON:
             symbols = symbols_section,
             connections = connections_section,
             imports = imports_section,
+            related = related_section,
             breaking = breaking_warning,
             constraints = constraints_section,
             focus = focus_instruction,

src/services/context.rs

@@ -230,6 +230,7 @@ impl ContextBuilder {
             history_context: None, // Set by App when learn_from_history is enabled
             connections: Self::detect_connections(changes, symbols),
             import_changes: Self::detect_import_changes(changes),
+            test_correlations: Self::detect_test_correlation(changes),
         }
     }
 
@@ -997,6 +998,37 @@ impl ContextBuilder {
         imports
     }
 
+    /// Detect source-to-test file relationships among staged changes.
+    fn detect_test_correlation(changes: &StagedChanges) -> Vec<String> {
+        let mut correlations = Vec::new();
+        let source_files: Vec<_> = changes
+            .files
+            .iter()
+            .filter(|f| f.category == FileCategory::Source)
+            .collect();
+        let test_files: Vec<_> = changes
+            .files
+            .iter()
+            .filter(|f| f.category == FileCategory::Test)
+            .collect();
+
+        for src in &source_files {
+            let src_stem = src.path.file_stem().and_then(|s| s.to_str()).unwrap_or("");
+            for test in &test_files {
+                let test_stem = test.path.file_stem().and_then(|s| s.to_str()).unwrap_or("");
+                if test_stem == src_stem || test_stem.starts_with(src_stem) {
+                    correlations.push(format!(
+                        "{} <-> {} (test file)",
+                        src.path.display(),
+                        test.path.display()
+                    ));
+                }
+            }
+        }
+        correlations.truncate(5);
+        correlations
+    }
+
     fn is_import_line(line: &str) -> bool {
         let trimmed = line.trim();
         trimmed.starts_with("use ")

tests/context.rs

@@ -1670,6 +1670,82 @@ fn semantic_only_change_has_no_suffix() {
     );
 }
 
+// ─── Test file correlation detection ──────────────────────────────────────────
+
+#[test]
+fn detect_test_file_correlation() {
+    let changes = make_staged_changes(vec![
+        make_file_change(
+            "src/services/context.rs",
+            ChangeStatus::Modified,
+            "+code\n",
+            1,
+            0,
+        ),
+        make_file_change("tests/context.rs", ChangeStatus::Modified, "+test\n", 1, 0),
+    ]);
+    let ctx = ContextBuilder::build(&changes, &[], &default_config());
+    assert_eq!(ctx.test_correlations.len(), 1);
+    assert!(ctx.test_correlations[0].contains("context"));
+}
+
+#[test]
+fn no_correlation_without_matching_test() {
+    let changes = make_staged_changes(vec![
+        make_file_change(
+            "src/services/context.rs",
+            ChangeStatus::Modified,
+            "+code\n",
+            1,
+            0,
+        ),
+        make_file_change("tests/other.rs", ChangeStatus::Modified, "+test\n", 1, 0),
+    ]);
+    let ctx = ContextBuilder::build(&changes, &[], &default_config());
+    assert!(ctx.test_correlations.is_empty());
+}
+
+#[test]
+fn test_correlation_shown_in_prompt() {
+    let changes = make_staged_changes(vec![
+        make_file_change(
+            "src/services/context.rs",
+            ChangeStatus::Modified,
+            "+code\n",
+            1,
+            0,
+        ),
+        make_file_change("tests/context.rs", ChangeStatus::Modified, "+test\n", 1, 0),
+    ]);
+    let ctx = ContextBuilder::build(&changes, &[], &default_config());
+    let prompt = ctx.to_prompt();
+    assert!(prompt.contains("RELATED FILES:"));
+}
+
+#[test]
+fn test_correlations_capped_at_5() {
+    let mut files = Vec::new();
+    for i in 0..8 {
+        files.push(make_file_change(
+            &format!("src/mod{i}.rs"),
+            ChangeStatus::Modified,
+            "+code\n",
+            1,
+            0,
+        ));
+        files.push(make_file_change(
+            &format!("tests/mod{i}.rs"),
+            ChangeStatus::Modified,
+            "+test\n",
+            1,
+            0,
+        ));
+    }
+    let changes = make_staged_changes(files);
+    let ctx = ContextBuilder::build(&changes, &[], &default_config());
+    assert!(ctx.test_correlations.len() <= 5);
+}
+
 #[test]
 fn python_comment_only_change_classified_as_docs() {
     let changes = make_staged_changes(vec![make_file_change(