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/crew/manager_test.go

@@ -16,7 +16,7 @@ func TestManagerAddAndGet(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to create temp dir: %v", err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() { _ = os.RemoveAll(tmpDir) }()
 
 	// Create a mock rig
 	rigPath := filepath.Join(tmpDir, "test-rig")
@@ -107,7 +107,7 @@ func TestManagerAddWithBranch(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to create temp dir: %v", err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() { _ = os.RemoveAll(tmpDir) }()
 
 	// Create a mock rig
 	rigPath := filepath.Join(tmpDir, "test-rig")
@@ -176,7 +176,7 @@ func TestManagerList(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to create temp dir: %v", err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() { _ = os.RemoveAll(tmpDir) }()
 
 	// Create a mock rig
 	rigPath := filepath.Join(tmpDir, "test-rig")
@@ -234,7 +234,7 @@ func TestManagerRemove(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to create temp dir: %v", err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() { _ = os.RemoveAll(tmpDir) }()
 
 	// Create a mock rig
 	rigPath := filepath.Join(tmpDir, "test-rig")