feat: platform-specific shell and agent launch helpers

DefaultShell, ExecAgent, ExecShell with prompt prefix. Banner, container notice. Tests for all helpers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/shell/launch.go

@@ -0,0 +1,106 @@
+package shell
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"runtime"
+)
+
+// DefaultShell returns the platform-appropriate interactive shell.
+func DefaultShell() string {
+	if runtime.GOOS == "windows" {
+		if _, err := exec.LookPath("powershell.exe"); err == nil {
+			return "powershell.exe"
+		}
+		return "cmd.exe"
+	}
+	shell := os.Getenv("SHELL")
+	if shell != "" {
+		return shell
+	}
+	return "bash"
+}
+
+// BuildEnvList merges ctask env vars into the current process environment.
+func BuildEnvList(vars map[string]string) []string {
+	env := os.Environ()
+	for k, v := range vars {
+		env = append(env, fmt.Sprintf("%s=%s", k, v))
+	}
+	return env
+}
+
+// PromptPrefix returns the shell prompt prefix for --shell mode.
+func PromptPrefix(slug, mode string) string {
+	return fmt.Sprintf("(ctask:%s|%s) ", slug, mode)
+}
+
+// BannerLines returns the launch banner lines for agent mode.
+func BannerLines(mode, slug, wsPath string) []string {
+	return []string{
+		fmt.Sprintf("[ctask] %s :: %s", mode, slug),
+		fmt.Sprintf("[ctask] %s", wsPath),
+	}
+}
+
+// ContainerNotice returns the v0.1 deferred container mode message.
+func ContainerNotice() string {
+	return "[ctask] container mode is not available in v0.1. Use local mode or see docs for manual container setup."
+}
+
+// ExecAgent launches the agent command in the workspace directory.
+func ExecAgent(agent string, wsDir string, envVars map[string]string) error {
+	path, err := exec.LookPath(agent)
+	if err != nil {
+		return fmt.Errorf("agent command not found: %s", agent)
+	}
+
+	cmd := exec.Command(path)
+	cmd.Dir = wsDir
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.Env = BuildEnvList(envVars)
+
+	return cmd.Run()
+}
+
+// ExecShell launches an interactive shell in the workspace directory.
+func ExecShell(wsDir string, envVars map[string]string, slug, mode string) error {
+	shellCmd := DefaultShell()
+	prefix := PromptPrefix(slug, mode)
+
+	var cmd *exec.Cmd
+	if runtime.GOOS == "windows" {
+		if shellCmd == "powershell.exe" {
+			cmd = exec.Command(shellCmd, "-NoExit", "-Command",
+				fmt.Sprintf("function prompt { '%s' + (Get-Location).Path + '> ' }", prefix))
+		} else {
+			// cmd.exe
+			cmd = exec.Command(shellCmd)
+		}
+	} else {
+		cmd = exec.Command(shellCmd)
+	}
+
+	cmd.Dir = wsDir
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+
+	env := BuildEnvList(envVars)
+
+	if runtime.GOOS == "windows" && shellCmd == "cmd.exe" {
+		env = append(env, fmt.Sprintf("PROMPT=%s$P$G", prefix))
+	} else if runtime.GOOS != "windows" {
+		existingPS1 := os.Getenv("PS1")
+		if existingPS1 == "" {
+			existingPS1 = "\\u@\\h:\\w\\$ "
+		}
+		env = append(env, fmt.Sprintf("PS1=%s%s", prefix, existingPS1))
+	}
+
+	cmd.Env = env
+	return cmd.Run()
+}