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/tools/git_test.go

@@ -0,0 +1,142 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func requireGit(t *testing.T) {
+	t.Helper()
+	if _, err := exec.LookPath("git"); err != nil {
+		t.Skip("git not installed")
+	}
+}
+
+func runGitForTest(t *testing.T, dir string, args ...string) {
+	t.Helper()
+	gitArgs := append([]string{"-C", dir}, args...)
+	cmd := exec.Command("git", gitArgs...)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("git %v failed: %v\n%s", args, err, out)
+	}
+}
+
+func initTestRepo(t *testing.T) string {
+	t.Helper()
+	dir := t.TempDir()
+	runGitForTest(t, dir, "init")
+	runGitForTest(t, dir, "-c", "user.email=test@example.com", "-c", "user.name=Test", "commit", "--allow-empty", "-m", "initial")
+	return dir
+}
+
+func TestGitStatusToolShowsChangedFiles(t *testing.T) {
+	requireGit(t)
+	dir := initTestRepo(t)
+
+	path := filepath.Join(dir, "tracked.txt")
+	if err := os.WriteFile(path, []byte("hello"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	runGitForTest(t, dir, "add", "tracked.txt")
+	runGitForTest(t, dir, "-c", "user.email=test@example.com", "-c", "user.name=Test", "commit", "-m", "add tracked")
+
+	if err := os.WriteFile(path, []byte("world"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	tool := NewGitStatusTool(dir)
+	out, err := tool.Execute(context.Background(), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(out, "Changed files:") {
+		t.Fatalf("output missing 'Changed files:': %q", out)
+	}
+	if !strings.Contains(out, "M tracked.txt") {
+		t.Fatalf("output missing 'M tracked.txt': %q", out)
+	}
+}
+
+func TestGitDiffToolShowsUnstagedDiff(t *testing.T) {
+	requireGit(t)
+	dir := initTestRepo(t)
+
+	path := filepath.Join(dir, "tracked.txt")
+	if err := os.WriteFile(path, []byte("hello"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	runGitForTest(t, dir, "add", "tracked.txt")
+	runGitForTest(t, dir, "-c", "user.email=test@example.com", "-c", "user.name=Test", "commit", "-m", "add tracked")
+
+	if err := os.WriteFile(path, []byte("world"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	tool := NewGitDiffTool(dir)
+	out, err := tool.Execute(context.Background(), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(out, "diff --git") {
+		t.Fatalf("output missing 'diff --git': %q", out)
+	}
+}
+
+func TestGitDiffToolShowsStat(t *testing.T) {
+	requireGit(t)
+	dir := initTestRepo(t)
+
+	path := filepath.Join(dir, "tracked.txt")
+	if err := os.WriteFile(path, []byte("hello"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	runGitForTest(t, dir, "add", "tracked.txt")
+	runGitForTest(t, dir, "-c", "user.email=test@example.com", "-c", "user.name=Test", "commit", "-m", "add tracked")
+
+	if err := os.WriteFile(path, []byte("world"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	tool := NewGitDiffTool(dir)
+	out, err := tool.Execute(context.Background(), json.RawMessage(`{"stat":true}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(out, "tracked.txt") {
+		t.Fatalf("output missing 'tracked.txt': %q", out)
+	}
+}
+
+func TestGitLogToolShowsCommits(t *testing.T) {
+	requireGit(t)
+	dir := initTestRepo(t)
+
+	tool := NewGitLogTool(dir)
+	out, err := tool.Execute(context.Background(), json.RawMessage(`{"max_count":1}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(out, "initial") {
+		t.Fatalf("output missing 'initial': %q", out)
+	}
+}
+
+func TestGitToolsRejectEscapingPath(t *testing.T) {
+	requireGit(t)
+	dir := initTestRepo(t)
+
+	tool := NewGitStatusTool(dir)
+	_, err := tool.Execute(context.Background(), json.RawMessage(`{"path":"../outside"}`))
+	if err == nil {
+		t.Fatal("expected error for escaping path")
+	}
+	if !strings.Contains(err.Error(), "escapes") {
+		t.Fatalf("error = %v", err)
+	}
+}