fix(executor): unset CLAUDECODE env to prevent nested session rejection

Claude Code v2.1.41+ sets CLAUDECODE=1 in all child Bash processes and
rejects startup when the variable is present. When codeagent-wrapper
spawns `claude -p` as a subprocess, it inherits this variable and gets
blocked with "cannot be launched inside another Claude Code session".

Add UnsetEnv method to commandRunner interface and strip CLAUDECODE
before spawning the claude backend.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

codeagent-wrapper/internal/app/main_test.go

@@ -274,6 +274,10 @@ func (d *drainBlockingCmd) Process() executor.ProcessHandle {
 	return d.inner.Process()
 }
 
+func (d *drainBlockingCmd) UnsetEnv(keys ...string) {
+	d.inner.UnsetEnv(keys...)
+}
+
 type bufferWriteCloser struct {
 	buf    bytes.Buffer
 	mu     sync.Mutex
@@ -568,6 +572,14 @@ func (f *fakeCmd) Process() executor.ProcessHandle {
 	return f.process
 }
 
+func (f *fakeCmd) UnsetEnv(keys ...string) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	for _, k := range keys {
+		delete(f.env, k)
+	}
+}
+
 func (f *fakeCmd) runStdoutScript() {
 	if len(f.stdoutPlan) == 0 {
 		if !f.keepStdoutOpen {