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/polecat/manager.go

@@ -109,7 +109,7 @@ func (m *Manager) Add(name string) (*Polecat, error) {
 	// Save state
 	if err := m.saveState(polecat); err != nil {
 		// Clean up worktree on failure
-		mayorGit.WorktreeRemove(polecatPath, true)
+		_ = mayorGit.WorktreeRemove(polecatPath, true)
 		return nil, fmt.Errorf("saving state: %w", err)
 	}
 
@@ -148,7 +148,7 @@ func (m *Manager) Remove(name string, force bool) error {
 	}
 
 	// Prune any stale worktree entries
-	mayorGit.WorktreePrune()
+	_ = mayorGit.WorktreePrune()
 
 	return nil
 }