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/git/git_test.go

@@ -21,10 +21,10 @@ func initTestRepo(t *testing.T) string {
 	// Configure user for commits
 	cmd = exec.Command("git", "config", "user.email", "test@test.com")
 	cmd.Dir = dir
-	cmd.Run()
+	_ = cmd.Run()
 	cmd = exec.Command("git", "config", "user.name", "Test User")
 	cmd.Dir = dir
-	cmd.Run()
+	_ = cmd.Run()
 
 	// Create initial commit
 	testFile := filepath.Join(dir, "README.md")
@@ -33,10 +33,10 @@ func initTestRepo(t *testing.T) string {
 	}
 	cmd = exec.Command("git", "add", ".")
 	cmd.Dir = dir
-	cmd.Run()
+	_ = cmd.Run()
 	cmd = exec.Command("git", "commit", "-m", "initial")
 	cmd.Dir = dir
-	cmd.Run()
+	_ = cmd.Run()
 
 	return dir
 }