M2: ai claude and codex CLI backends with selector

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/ai/backend.go

@@ -0,0 +1,101 @@
+package ai
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+// claudeBackend shells out to `claude --print` with the prompt on stdin and the
+// answer on stdout.
+type claudeBackend struct {
+	cmd  string
+	args []string
+}
+
+func newClaudeBackend() claudeBackend {
+	return claudeBackend{
+		cmd:  "claude",
+		args: []string{"--print", "--tools", "", "--no-session-persistence", "--output-format", "text"},
+	}
+}
+
+func (b claudeBackend) Name() string { return "claude" }
+
+func (b claudeBackend) Run(ctx context.Context, prompt string) (string, error) {
+	cmd := exec.CommandContext(ctx, b.cmd, b.args...)
+	cmd.Stdin = strings.NewReader(prompt)
+	var out, errb bytes.Buffer
+	cmd.Stdout = &out
+	cmd.Stderr = &errb
+	if err := cmd.Run(); err != nil {
+		return "", fmt.Errorf("claude: %w: %s", err, strings.TrimSpace(errb.String()))
+	}
+	return out.String(), nil
+}
+
+// codexBackend shells out to `codex exec` headlessly. codex's stdout includes
+// session preamble, so the final answer is written to a temp file via -o and
+// read back. The --ignore-* and read-only flags stop it from running shell
+// commands driven by local config.
+type codexBackend struct {
+	cmd      string
+	baseArgs []string
+}
+
+func newCodexBackend() codexBackend {
+	return codexBackend{
+		cmd: "codex",
+		baseArgs: []string{
+			"exec", "--skip-git-repo-check", "--ignore-user-config",
+			"--ignore-rules", "-s", "read-only", "-a", "never", "--ephemeral",
+		},
+	}
+}
+
+func (b codexBackend) Name() string { return "codex" }
+
+// args builds the full argument list, writing the answer to outfile and reading
+// the prompt from stdin (trailing "-").
+func (b codexBackend) args(outfile string) []string {
+	out := append([]string{}, b.baseArgs...)
+	return append(out, "-o", outfile, "-")
+}
+
+func (b codexBackend) Run(ctx context.Context, prompt string) (string, error) {
+	f, err := os.CreateTemp("", "antidrift-codex-*.out")
+	if err != nil {
+		return "", fmt.Errorf("codex: temp file: %w", err)
+	}
+	path := f.Name()
+	_ = f.Close()
+	defer os.Remove(path)
+
+	cmd := exec.CommandContext(ctx, b.cmd, b.args(path)...)
+	cmd.Stdin = strings.NewReader(prompt)
+	var errb bytes.Buffer
+	cmd.Stderr = &errb // stdout is noisy and intentionally discarded
+	if err := cmd.Run(); err != nil {
+		return "", fmt.Errorf("codex: %w: %s", err, strings.TrimSpace(errb.String()))
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return "", fmt.Errorf("codex: read output: %w", err)
+	}
+	return string(data), nil
+}
+
+// NewBackend returns the named backend. "" defaults to "claude".
+func NewBackend(name string) (Backend, error) {
+	switch name {
+	case "", "claude":
+		return newClaudeBackend(), nil
+	case "codex":
+		return newCodexBackend(), nil
+	default:
+		return nil, fmt.Errorf("ai: unknown backend %q", name)
+	}
+}