feat: add breaking_change instruction and field to user prompt template

Extends the user prompt in to_prompt() to include breaking change
detection guidance and adds the breaking_change field to the output
format template, aligning it with the updated SYSTEM_PROMPT schema.

Author: Sephyi

src/domain/context.rs

@@ -32,9 +32,10 @@ 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.
+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.
 
 Output format:
-{{"type": "{commit_type}", "scope": {scope_json}, "subject": "<imperative verb + what changed>", "body": "<why this change was made, or null if trivial>"}}"#,
+{{"type": "{commit_type}", "scope": {scope_json}, "subject": "<imperative verb + what changed>", "body": "<why this change was made, or null if trivial>", "breaking_change": null}}"#,
             summary = self.change_summary,
             files = self.file_breakdown.trim(),
             commit_type = self.suggested_type.as_str(),