Suppress conversational preambles in output

internal/config/assets/conventional.md

@@ -12,8 +12,3 @@ Rules:
   - Describe the breaking change clearly in the footer using "BREAKING CHANGE:"
 - If useful, add a body explaining *why* the change was made, not just what changed
 - Wrap body lines at approximately 72 characters
-- Do not mention the diff, filenames, or line numbers explicitly
-- Do not wrap the message in backticks or code fences
-- Output only the commit message, with no extra commentary
-
-Write the commit message based on the following git diff:

internal/config/assets/default.md

@@ -20,8 +20,3 @@ Rules:
   - Use full sentences and paragraphs as needed
 
 - Do not include metadata such as timestamps or author names
-- Do not mention the diff, file names, or line numbers explicitly
-- Do not wrap the message in backticks or code fences
-- Output only the commit message, with no additional explanations
-
-Write the commit message based on the following git diff:

internal/config/assets/gitmoji.md

@@ -26,8 +26,3 @@ Rules:
 - Use only one emoji per commit
 - Do not include Conventional Commits types (feat, fix, etc.)
 - Do not mix multiple conventions in a single commit
-- Do not mention the diff, file names, or line numbers explicitly
-- Do not wrap the message in backticks or code fences
-- Output only the commit message, with no explanations or metadata
-
-Write the commit message based on the following git diff:

internal/config/assets/karma.md

@@ -34,9 +34,3 @@ Rules:
 - If the change introduces a breaking change:
   - Use BREAKING CHANGE in the footer
   - The subject must describe the change, not the migration steps
-
-- Do not reference the diff, file names, or line numbers directly
-- Do not wrap the message in backticks or code fences
-- Output only the commit message, without any explanations or metadata
-
-Write the commit message based on the following git diff:

internal/prompt/prompt.go

@@ -1,20 +1,33 @@
 package prompt
 
-import "strings"
+import (
+	"bytes"
+	_ "embed"
+	"strings"
+	"text/template"
+)
+
+//go:embed prompt.tmpl
+var promptTemplateText string
+
+var promptTemplate = template.Must(template.New("prompt").Parse(promptTemplateText))
+
+type PromptData struct {
+	SystemPrompt string
+	Context      string
+	Diff         string
+}
 
 func Build(systemPrompt, context, diff string) string {
-	var b strings.Builder
-	if systemPrompt != "" {
-		b.WriteString("System Prompt:\n")
-		b.WriteString(systemPrompt)
-		b.WriteString("\n\n")
+	data := PromptData{
+		SystemPrompt: strings.TrimSpace(systemPrompt),
+		Context:      strings.TrimSpace(context),
+		Diff:         diff,
 	}
-	if context != "" {
-		b.WriteString("Context:\n")
-		b.WriteString(context)
-		b.WriteString("\n\n")
+	var buf bytes.Buffer
+	if err := promptTemplate.Execute(&buf, data); err != nil {
+		// Fallback to simple concatenation on template error
+		return systemPrompt + "\n\n" + context + "\n\n" + diff
 	}
-	b.WriteString("Git Diff:\n")
-	b.WriteString(diff)
-	return b.String()
+	return buf.String()
 }

internal/prompt/prompt.tmpl

@@ -0,0 +1,19 @@
+=== INSTRUCTIONS ===
+{{.SystemPrompt}}
+
+OUTPUT RULES:
+- Output only a Git commit message
+- Begin with the commit subject line
+- Exclude explanations, preambles, and meta commentary
+- DO NOT reference diffs, file names, or line numbers
+- DO NOT use code fences or backticks
+
+{{if .Context}}
+=== CONTEXT ===
+{{.Context}}
+{{end}}
+
+=== GIT DIFF ===
+{{.Diff}}
+
+=== OUTPUT ===

internal/prompt/prompt_test.go

@@ -1,11 +1,63 @@
 package prompt
 
-import "testing"
+import (
+	"strings"
+	"testing"
+)
 
 func TestBuild(t *testing.T) {
 	got := Build("sys", "ctx", "diff")
-	expected := "System Prompt:\nsys\n\nContext:\nctx\n\nGit Diff:\ndiff"
-	if got != expected {
-		t.Fatalf("Build = %q", got)
+
+	// Check that output contains expected sections
+	if !strings.Contains(got, "=== INSTRUCTIONS ===") {
+		t.Fatal("Build output should contain INSTRUCTIONS section")
+	}
+	if !strings.Contains(got, "sys") {
+		t.Fatal("Build output should contain system prompt")
+	}
+	if !strings.Contains(got, "OUTPUT RULES:") {
+		t.Fatal("Build output should contain OUTPUT RULES section")
+	}
+	if !strings.Contains(got, "=== CONTEXT ===") {
+		t.Fatal("Build output should contain CONTEXT section when context provided")
+	}
+	if !strings.Contains(got, "ctx") {
+		t.Fatal("Build output should contain context content")
+	}
+	if !strings.Contains(got, "=== GIT DIFF ===") {
+		t.Fatal("Build output should contain GIT DIFF section")
+	}
+	if !strings.Contains(got, "diff") {
+		t.Fatal("Build output should contain diff content")
+	}
+	if !strings.Contains(got, "=== OUTPUT ===") {
+		t.Fatal("Build output should contain OUTPUT section")
+	}
+}
+
+func TestBuildWithoutContext(t *testing.T) {
+	got := Build("sys", "", "diff")
+
+	// Check that output omits CONTEXT section when empty
+	if strings.Contains(got, "=== CONTEXT ===") {
+		t.Fatal("Build output should not contain CONTEXT section when context is empty")
+	}
+	if !strings.Contains(got, "=== INSTRUCTIONS ===") {
+		t.Fatal("Build output should contain INSTRUCTIONS section")
+	}
+	if !strings.Contains(got, "=== GIT DIFF ===") {
+		t.Fatal("Build output should contain GIT DIFF section")
+	}
+}
+
+func TestBuildOutputRules(t *testing.T) {
+	got := Build("sys", "ctx", "diff")
+
+	// Verify output rules are included
+	if !strings.Contains(got, "Exclude explanations, preambles") {
+		t.Fatal("Build output should contain preamble suppression rule")
+	}
+	if !strings.Contains(got, "Begin with the commit subject line") {
+		t.Fatal("Build output should contain direct start instruction")
 	}
 }