feat: Phase 1 code-agent capabilities — file_edit, git tools, parallel execution, tool output preview

New tools:
- file_edit: surgical line-range or pattern edits (yolo only)
- git_status: show changed files (read-only)
- git_diff: show unstaged/staged diff (read-only)
- git_log: show recent commits (read-only)

Agent improvements:
- parallel execution for independent read-only tool calls
- mutating tools (file_write, file_edit, shell_run) run sequentially
- tool output preview in logs with [output] marker

TUI improvements:
- tool cards now show truncated output preview (up to 8 lines)
- mergeToolLog updates placeholder cards with results
- toolStatusText keeps activity bar single-line

Documentation:
- README updated with git tools and file_edit descriptions

internal/tui/app.go

@@ -290,8 +290,10 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 	case toolLogMsg:
 		text := cleanToolLog(string(msg))
 		if text != "" {
-			m.messages = append(m.messages, message{role: roleTool, content: text})
-			m.status = text
+			if !m.mergeToolLog(text) {
+				m.messages = append(m.messages, message{role: roleTool, content: text})
+			}
+			m.status = toolStatusText(text)
 			m.refreshViewport(true)
 		}
 		return m, m.waitForEvent()
@@ -572,8 +574,9 @@ func roleLabel(r role) string {
 }
 
 type toolLogParts struct {
-	name string
-	args string
+	name   string
+	args   string
+	output string
 }
 
 func parseToolLog(content string) toolLogParts {
@@ -581,14 +584,21 @@ func parseToolLog(content string) toolLogParts {
 	if content == "" {
 		return toolLogParts{}
 	}
-	idx := strings.IndexFunc(content, unicode.IsSpace)
+	beforeOutput, output, hasOutput := strings.Cut(content, "\n[output]\n")
+	parts := toolLogParts{}
+	if hasOutput {
+		parts.output = strings.TrimSpace(output)
+	}
+	firstLine := strings.SplitN(beforeOutput, "\n", 2)[0]
+	firstLine = strings.TrimSpace(firstLine)
+	idx := strings.IndexFunc(firstLine, unicode.IsSpace)
 	if idx < 0 {
-		return toolLogParts{name: content}
-	}
-	return toolLogParts{
-		name: strings.TrimSpace(content[:idx]),
-		args: strings.TrimSpace(content[idx+1:]),
+		parts.name = firstLine
+		return parts
 	}
+	parts.name = strings.TrimSpace(firstLine[:idx])
+	parts.args = strings.TrimSpace(firstLine[idx+1:])
+	return parts
 }
 
 func (m model) toolMessageView(content string, width int) string {
@@ -599,11 +609,60 @@ func (m model) toolMessageView(content string, width int) string {
 		args := fitLine(parts.args, max(1, width-4))
 		lines = append(lines, m.styles.ToolValue.Render(args))
 	}
+	if parts.output != "" {
+		lines = append(lines, m.styles.ToolKey.Render("output"))
+		previewLines := toolOutputPreviewLines(parts.output, max(1, width-4))
+		for _, line := range previewLines {
+			lines = append(lines, m.styles.ToolValue.Render(line))
+		}
+	}
 	bodyContent := strings.Join(lines, "\n")
 	bodyWidth := messageBodyWidth(bodyContent, width, m.styles.ToolMessage)
 	return m.styles.ToolMessage.Width(bodyWidth).Render(bodyContent)
 }
 
+const maxToolOutputPreviewLines = 8
+
+func toolStatusText(content string) string {
+	content = strings.TrimSpace(content)
+	if content == "" {
+		return ""
+	}
+	lines := strings.SplitN(content, "\n", 2)
+	return lines[0]
+}
+
+func (m *model) mergeToolLog(text string) bool {
+	incoming := parseToolLog(text)
+	if incoming.output == "" {
+		return false
+	}
+	for i := len(m.messages) - 1; i >= 0; i-- {
+		if m.messages[i].role != roleTool {
+			continue
+		}
+		existing := parseToolLog(m.messages[i].content)
+		if existing.name == incoming.name && existing.args == incoming.args && existing.output == "" {
+			m.messages[i].content = text
+			return true
+		}
+	}
+	return false
+}
+
+func toolOutputPreviewLines(output string, width int) []string {
+	lines := strings.Split(output, "\n")
+	if len(lines) > maxToolOutputPreviewLines {
+		lines = lines[:maxToolOutputPreviewLines]
+		lines = append(lines, fmt.Sprintf("[truncated: showing first %d lines]", maxToolOutputPreviewLines))
+	}
+	result := make([]string, 0, len(lines))
+	for _, line := range lines {
+		result = append(result, fitLine(line, width))
+	}
+	return result
+}
+
 type metaPill struct {
 	text   string
 	accent bool