feat(v0.5): launch agent inside project subdirectory via launch_dir

LaunchOpts gains LaunchDir. session.Run resolves it via
workspace.ResolveLaunch, prints any fallback warning, and passes the
absolute path as the child process's working directory. Security
violations (absolute paths, .. escape) abort the session. The banner
gains a 'project dir: <name>/' line when launch_dir is set.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/session/run.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/warrenronsiek/ctask/internal/lockfile"
 	"github.com/warrenronsiek/ctask/internal/shell"
+	"github.com/warrenronsiek/ctask/internal/workspace"
 )
 
 // LaunchOpts configures a session launch.
@@ -20,6 +21,12 @@ type LaunchOpts struct {
 	Slug    string
 	Shell   bool // true = interactive shell, false = agent
 
+	// LaunchDir is the workspace-relative launch directory (v0.5). Empty for
+	// tasks and pre-v0.5 projects. When set, Run resolves the absolute path
+	// via workspace.ResolveLaunch and uses it as the child's working dir;
+	// a security violation (absolute path or .. escape) aborts the session.
+	LaunchDir string
+
 	// Force suppresses both the active-session warning (Layer 1) and the
 	// stale-workspace warning (Layer 3). It does NOT disable the metadata
 	// write lock or the session summary. Used for scripted/automated runs.
@@ -138,15 +145,27 @@ func Run(opts LaunchOpts) error {
 		hb = StartHeartbeat(leasePath, HeartbeatInterval)
 	}
 
+	// ---- Resolve launch directory (v0.5) ----
+	// Security violations (absolute path, ..-escape) abort the session
+	// before we hand control to the child. Missing/non-dir falls back to
+	// wsDir with a warning.
+	launchAbs, launchWarn, launchErr := workspace.ResolveLaunch(opts.WsDir, opts.LaunchDir)
+	if launchErr != nil {
+		return fmt.Errorf("resolving launch_dir: %w", launchErr)
+	}
+	if launchWarn != "" {
+		fmt.Fprintln(os.Stderr, launchWarn)
+	}
+
 	// ---- Run the child ----
 	var childErr error
 	if opts.Shell {
-		childErr = shell.ExecShell(opts.WsDir, opts.EnvVars, opts.Slug, opts.Mode)
+		childErr = shell.ExecShell(launchAbs, opts.EnvVars, opts.Slug, opts.Mode)
 	} else {
-		for _, line := range shell.BannerLines(opts.Mode, opts.Slug, opts.WsDir) {
+		for _, line := range shell.BannerLines(opts.Mode, opts.Slug, opts.WsDir, opts.LaunchDir) {
 			fmt.Println(line)
 		}
-		childErr = shell.ExecAgent(opts.Agent, opts.WsDir, opts.EnvVars)
+		childErr = shell.ExecAgent(opts.Agent, launchAbs, opts.EnvVars)
 	}
 
 	if hb != nil {