fix(lint): resolve all errcheck warnings

Fix ~50 errcheck warnings across the codebase:

- Add explicit `_ =` for intentionally ignored error returns (cleanup,
  best-effort operations, etc.)
- Use `defer func() { _ = ... }()` pattern for defer statements
- Handle tmux SetEnvironment, KillSession, SendKeysRaw returns
- Handle mail router.Send returns
- Handle os.RemoveAll, os.Rename in cleanup paths
- Handle rand.Read returns for ID generation
- Handle fmt.Fprint* returns when writing to io.Writer
- Fix for-select with single case to use for-range
- Handle cobra MarkFlagRequired returns

All tests pass. Code compiles without errors.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/refinery/engineer_test.go

@@ -36,7 +36,7 @@ func TestEngineer_LoadConfig_NoFile(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() { _ = os.RemoveAll(tmpDir) }()
 
 	r := &rig.Rig{
 		Name: "test-rig",
@@ -62,7 +62,7 @@ func TestEngineer_LoadConfig_WithMergeQueue(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() { _ = os.RemoveAll(tmpDir) }()
 
 	// Write config file
 	config := map[string]interface{}{
@@ -124,7 +124,7 @@ func TestEngineer_LoadConfig_NoMergeQueueSection(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() { _ = os.RemoveAll(tmpDir) }()
 
 	// Write config file without merge_queue
 	config := map[string]interface{}{
@@ -160,7 +160,7 @@ func TestEngineer_LoadConfig_InvalidPollInterval(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() { _ = os.RemoveAll(tmpDir) }()
 
 	config := map[string]interface{}{
 		"merge_queue": map[string]interface{}{