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/executor/env_stderr_test.go

@@ -41,6 +41,11 @@ func (f *fakeCmd) SetEnv(env map[string]string) {
 	}
 }
 func (f *fakeCmd) Process() processHandle { return nil }
+func (f *fakeCmd) UnsetEnv(keys ...string) {
+	for _, k := range keys {
+		delete(f.env, k)
+	}
+}
 
 func TestEnvInjection_LogsToStderrAndMasksKey(t *testing.T) {
 	// Arrange ~/.codeagent/models.json via HOME override.