feat(context): rebalance token budget for structural diffs and update SYSTEM_PROMPT

Reduce symbol budget from 30% to 20% when structural diffs are present,
freeing space for the raw diff. Add SYSTEM_PROMPT guidance to prefer
STRUCTURED CHANGES for signature-level details over raw diff lines.

Author: Sephyi

src/services/context.rs

@@ -51,8 +51,13 @@ impl ContextBuilder {
         let used = SYSTEM_PROMPT_RESERVE + change_summary.len() + file_breakdown.len();
         let remaining = max_context.saturating_sub(used);
 
-        // Symbols get 30% when signatures are present (richer content), 20% otherwise
-        let symbol_pct = if symbols.iter().any(|s| s.signature.is_some()) {
+        // When structural diffs are available, symbols need less budget (diffs carry the detail).
+        // When only signatures are available, symbols still get 30%.
+        // Base case without signatures: 20%.
+        let has_structural_diffs = !diffs.is_empty();
+        let symbol_pct = if has_structural_diffs {
+            20
+        } else if symbols.iter().any(|s| s.signature.is_some()) {
             30
         } else {
             20

src/services/llm/mod.rs

@@ -29,6 +29,7 @@ Rules:
 - Never copy labels, field names, or evidence tags from the prompt into your output.
 - If public APIs are both added and removed, this is an API replacement (refactor), not a new feature.
 - When SYMBOLS CHANGED shows full signatures, reference the actual parameter/type names in your subject rather than generic descriptions.
+- When STRUCTURED CHANGES shows parameter/return/visibility changes, use those specifics in the subject (e.g., "add timeout parameter to connect function"). STRUCTURED CHANGES is the most reliable source for what changed in function signatures — prefer it over reading raw diff lines.
 - When CONNECTIONS shows that a caller and callee both changed, consider mentioning the relationship in the body.
 
 Symbol markers: [+] added, [-] removed, [~] modified (signature changed).

tests/context.rs

@@ -1827,3 +1827,43 @@ fn python_comment_only_change_classified_as_docs() {
         "Python comment-only change should suggest Docs"
     );
 }
+
+// ─── Token budget rebalancing with structural diffs ───────────────────────────
+
+#[test]
+fn symbol_budget_reduced_when_structural_diffs_present() {
+    use commitbee::domain::diff::{ChangeDetail, SymbolDiff};
+
+    let changes = make_staged_changes(vec![make_file_change(
+        "src/lib.rs",
+        ChangeStatus::Modified,
+        &"+pub fn foo() {}\n".repeat(50),
+        50,
+        0,
+    )]);
+    let mut sym = make_symbol("foo", SymbolKind::Function, "src/lib.rs", true, true);
+    sym.signature = Some("pub fn foo()".into());
+    let diffs = vec![SymbolDiff {
+        name: "foo".into(),
+        file: "src/lib.rs".into(),
+        line: 1,
+        parent_scope: None,
+        changes: vec![ChangeDetail::BodyModified {
+            additions: 5,
+            deletions: 2,
+        }],
+    }];
+
+    // With diffs: symbol budget should be reduced (20%)
+    let ctx_with = ContextBuilder::build(&changes, &[sym.clone()], &diffs, &default_config());
+    // Without diffs: symbol budget at 30% (signatures present)
+    let ctx_without = ContextBuilder::build(&changes, &[sym], &[], &default_config());
+
+    // The diff section should be larger when structural diffs reduce symbol budget
+    assert!(
+        ctx_with.truncated_diff.len() >= ctx_without.truncated_diff.len(),
+        "structural diffs should free budget for raw diff: with={} without={}",
+        ctx_with.truncated_diff.len(),
+        ctx_without.truncated_diff.len()
+    );
+}