refactor(llm): simplify prompts, add concrete entity rule, fix type defaults

Removes Think-then-Compress instructions that caused token budget
exhaustion on small models. Integrates concrete entity naming rule into
system prompt Subject line. Simplifies user prompt to concise format.
Adds bug evidence → fix inference and changes default fallback from
Feat to Refactor.

Author: Sephyi

src/domain/context.rs

@@ -89,13 +89,11 @@ SUGGESTED TYPE: {commit_type}{scope}
 DIFF:
 {diff}
 {constraints}{breaking}{metadata_breaking}{focus}
-Write a JSON commit message describing the changes shown in the diff.
-The subject must be specific and under {subject_budget} chars — 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.
-For breaking_change: only set this if existing users or dependents must change their code, config, or scripts to keep working — e.g., a public function/endpoint removed or renamed, a required parameter or field added, a config key changed. New optional features, bug fixes, and internal refactors are NOT breaking. Default to null.
+Subject must be under {subject_budget} chars and name at least one concrete entity (function, struct, variable) from the diff.
+Body: 1-3 sentences on WHY, or null if trivial. breaking_change: only if existing users must change code/config to stay compatible, else null.
 
-Output format:
-{{"type": "<type>", "scope": {scope_json}, "subject": "<imperative verb + what changed>", "body": "<why this change was made, or null if trivial>", "breaking_change": null}}"#,
+Respond with ONLY this JSON:
+{{"type": "<type>", "scope": {scope_json}, "subject": "<imperative verb + what changed>", "body": null, "breaking_change": null}}"#,
             summary = self.change_summary,
             files = self.file_breakdown.trim(),
             commit_type = self.suggested_type.as_str(),

src/services/context.rs

@@ -168,6 +168,11 @@ impl ContextBuilder {
             return CommitType::Build;
         }
 
+        // Explicit bug evidence -> fix
+        if Self::detect_bug_evidence(changes) {
+            return CommitType::Fix;
+        }
+
         // New public functions/structs -> feat (unless it's an API replacement)
         let has_new_public_symbols = symbols.iter().any(|s| {
             s.is_added
@@ -226,7 +231,7 @@ impl ContextBuilder {
             return CommitType::Refactor;
         }
 
-        CommitType::Feat
+        CommitType::Refactor
     }
 
     pub fn infer_scope(changes: &StagedChanges) -> Option<String> {

src/services/llm/mod.rs

@@ -17,26 +17,25 @@ parameter added, a config key renamed. New optional additions, bug fixes, and in
 refactors are NOT breaking. Default to null.
 
 Rules:
-- Subject: imperative, specific, lowercase start, no trailing period, max 72 chars total first line.
+- Subject: imperative, specific, lowercase start, no trailing period, max 72 chars total first line. Must name at least one concrete entity (function, struct, variable, file) from the diff.
 - Body: 1-3 sentences about WHY for non-trivial changes, else null.
 - Do not list files changed.
-- If the change is purely syntactic (collapsing nested blocks, reformatting, reordering imports) with identical behavior, use "style" — never describe it as fixing a bug or adding a feature.
+- If the change is purely syntactic (reformatting, reordering imports) with identical behavior, use "style".
 - The SUGGESTED TYPE is a hint. Override it if the diff clearly shows a different type.
-- Never copy labels, field names, or evidence tags from the prompt into your output. The breaking_change value must describe the actual change in plain English.
+- 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.
 
 Examples:
-GOOD: "add evidence-based commit validation with retry" — one specific thing
-GOOD: "replace path-only grouping with diff-shape fingerprinting" — names the concrete change
-BAD: "update code and improve things" — too vague
-BAD: "refactor code for better performance and add validation" — two concerns in one subject
+GOOD: "replace path-only grouping with diff-shape fingerprinting"
+GOOD: "add CommitValidator for evidence-based retry"
+BAD: "update code and improve things" — too generic
+BAD: "refactor code for better performance and add validation" — two concerns
 
 1. Diff adds `pub fn new_api()` and removes `pub fn old_api()` → type: "refactor", breaking_change: "removed `old_api()`, use `new_api()` instead"
 2. Diff only changes whitespace/indentation → type: "style", body: null, breaking_change: null
 
-Output ONLY valid JSON (nullable fields use null, not the string "null"):
+Respond with ONLY the JSON object, nothing else:
 {"type":"<type>","scope":null,"subject":"<subject>","body":null,"breaking_change":null}
-For scope, body, and breaking_change: replace null with a quoted string when applicable.
 "#;
 
 pub mod anthropic;

tests/context.rs

@@ -394,7 +394,7 @@ fn infer_type_more_deletions_is_refactor() {
 }
 
 #[test]
-fn infer_type_default_fallback_is_feat() {
+fn infer_type_default_fallback_is_refactor() {
     // 30 insertions, 30 deletions (not small, not deletion-heavy, not special category)
     let changes = make_staged_changes(vec![make_file_change(
         "src/services/module.rs",
@@ -406,8 +406,8 @@ fn infer_type_default_fallback_is_feat() {
     let ctx = ContextBuilder::build(&changes, &[], &default_config());
     assert_eq!(
         ctx.suggested_type,
-        CommitType::Feat,
-        "large non-special change should fallback to Feat"
+        CommitType::Refactor,
+        "large non-special change should fallback to Refactor (safer than Feat)"
     );
 }