refactor: LLM 直接输出 Markdown，移除 JSON 解析层

服务端:
- prompt 改为要求直接输出 Markdown，不再输出 JSON
- GeneratePR 返回 string 而非 PRDescription 结构体
- 移除 JSON 解析、extractJSON、fixInlineCode 等逻辑
- ChatStream 的 content 事件直接携带 Markdown 片段流式传输

前端:
- generate() 仅处理 content + done + error 三个事件
- content 事件累积 Markdown 文本，实时渲染到两栏
- 移除 parsePartialJSON、buildMarkdown 等中间层
- 代码量从 ~100 行减至 ~50 行

services/generate.go

@@ -2,9 +2,7 @@ package services
 
 import (
 	"database/sql"
-	"encoding/json"
 	"fmt"
-	"regexp"
 	"strings"
 
 	"github.com/go-git/go-git/v5/plumbing"
@@ -12,34 +10,25 @@ import (
 	goopenai "github.com/sashabaranov/go-openai"
 )
 
-// PRDescription is the structured output from PR description generation.
-type PRDescription struct {
-	Title   string `json:"title"`
-	Type    string `json:"type"`
-	Summary string `json:"summary"`
-	Details string `json:"details"`
-	Impact  string `json:"impact"`
-}
-
-// GeneratePR generates a structured PR description from commit history and diff.
-// It streams progress via the callback and returns the parsed PR description.
-func GeneratePR(db *sql.DB, repoPath, base, head string, userID int64, callback StreamCallback) (*PRDescription, error) {
+// GeneratePR generates a PR description as Markdown from commit history and diff.
+// It streams the LLM output via callback ("content" events) and returns the full markdown.
+func GeneratePR(db *sql.DB, repoPath, base, head string, userID int64, callback StreamCallback) (string, error) {
 	// Read LLM config (per-user)
 	config, err := GetLLMConfig(db, userID)
 	if err != nil {
-		return nil, err
+		return "", err
 	}
 
 	// Open repo
 	repo, err := OpenRepo(repoPath)
 	if err != nil {
-		return nil, fmt.Errorf("open repo: %w", err)
+		return "", fmt.Errorf("open repo: %w", err)
 	}
 
 	// Get commits between base and head
 	commits, err := GetCommitLog(repo, head, 100)
 	if err != nil {
-		return nil, fmt.Errorf("get commits: %w", err)
+		return "", fmt.Errorf("get commits: %w", err)
 	}
 
 	// Filter commits to only those reachable from head but not from base
@@ -78,7 +67,7 @@ func GeneratePR(db *sql.DB, repoPath, base, head string, userID int64, callback
 	// Get diff
 	diff, err := GetDiff(repo, base, head)
 	if err != nil {
-		return nil, fmt.Errorf("get diff: %w", err)
+		return "", fmt.Errorf("get diff: %w", err)
 	}
 
 	// Build prompt
@@ -97,7 +86,7 @@ func GeneratePR(db *sql.DB, repoPath, base, head string, userID int64, callback
 		diff = diff[:60000] + "\n\n... [diff truncated due to size]"
 	}
 
-	prompt := fmt.Sprintf(`你是一个专业的技术文档撰写助手。根据以下 Git 变更信息，生成一份结构化的 PR 描述。
+	prompt := fmt.Sprintf(`你是一个专业的技术文档撰写助手。根据以下 Git 变更信息，生成一份 PR 描述。
 
 ## Commit 记录
 %s
@@ -105,65 +94,40 @@ func GeneratePR(db *sql.DB, repoPath, base, head string, userID int64, callback
 ## 代码变更 (Diff)
 %s
 
-请按以下 JSON 格式输出（直接输出 JSON，不要包含 markdown 代码块标记）：
-{
-  "title": "简洁的 PR 标题",
-  "type": "变更类型: feat|fix|refactor|docs|chore|style|test|perf",
-  "summary": "一段话概述变更内容",
-  "details": "详细的变更说明，按模块分组，使用 Markdown 格式",
-  "impact": "影响范围说明"
-}
+请直接输出 Markdown 格式的 PR 描述，包含以下部分：
 
-重要格式要求：
-- details 字段中引用文件名、函数名、变量名等代码标识时，必须用反引号包裹，例如：`+"`services/generate.go`"+`, `+"`GeneratePR()`"+`
-- 直接写出实际的代码名称，不要用任何占位符替代`, commitStr, diff)
+# 标题
+
+**类型**: feat|fix|refactor|docs|chore|style|test|perf
+
+## 概述
+一段话概述变更内容
+
+## 详细说明
+按模块分组的详细变更说明
+
+## 影响范围
+影响范围说明
+
+格式要求：
+- 引用文件名、函数名、变量名等代码标识时，必须用反引号包裹
+- 直接写出实际的代码名称，不要用任何占位符替代
+- 不要输出 JSON，直接输出 Markdown`, commitStr, diff)
 
 	messages := []goopenai.ChatCompletionMessage{
 		{Role: goopenai.ChatMessageRoleUser, Content: prompt},
 	}
 
-	// Call LLM with streaming
+	// Call LLM with streaming — content events carry markdown chunks in real-time
 	fullResponse, err := ChatStream(config, messages, callback)
 	if err != nil {
-		return nil, fmt.Errorf("LLM call: %w", err)
+		return "", fmt.Errorf("LLM call: %w", err)
 	}
 
-	// Parse JSON response
-	jsonStr := extractJSON(fullResponse)
-	var pr PRDescription
-	if err := json.Unmarshal([]byte(jsonStr), &pr); err != nil {
-		return nil, fmt.Errorf("parse PR description: %w (raw: %s)", err, truncateString(fullResponse, 200))
-	}
-
-	// Post-process: convert INLINECODE placeholders back to backtick-enclosed code
-	pr.Details = fixInlineCode(pr.Details)
-
-	// Send structured events
+	// Signal completion
 	if callback != nil {
-		callback("title", map[string]interface{}{"content": pr.Title})
-		callback("type", map[string]interface{}{"content": pr.Type})
-		callback("summary", map[string]interface{}{"content": pr.Summary})
-		callback("detail", map[string]interface{}{"content": pr.Details})
-		callback("impact", map[string]interface{}{"content": pr.Impact})
 		callback("done", map[string]interface{}{"content": ""})
 	}
 
-	return &pr, nil
-}
-
-// fixInlineCode converts LLM-generated INLINECODE placeholders back to backtick-enclosed inline code.
-// Some LLMs output INLINECODE0, INLINECODE1, etc. instead of `code` in JSON string values.
-// This wraps them in backticks so the frontend markdown renderer displays them as inline code.
-func fixInlineCode(text string) string {
-	re := regexp.MustCompile(`(?i)INLINECODE[_]?(\d+)`)
-	return re.ReplaceAllStringFunc(text, func(match string) string {
-		return "`" + match + "`"
-	})
-}
-
-func truncateString(s string, maxLen int) string {
-	if len(s) <= maxLen {
-		return s
-	}
-	return s[:maxLen] + "..."
+	return fullResponse, nil
 }