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/rig/manager.go

@@ -194,7 +194,7 @@ func (m *Manager) AddRig(opts AddRigOptions) (*Rig, error) {
 	}
 
 	// Track cleanup on failure
-	cleanup := func() { os.RemoveAll(rigPath) }
+	cleanup := func() { _ = os.RemoveAll(rigPath) }
 	success := false
 	defer func() {
 		if !success {