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/doctor/types.go

@@ -141,7 +141,7 @@ func (r *Report) Print(w io.Writer, verbose bool) {
 	}
 
 	// Print summary
-	fmt.Fprintln(w)
+	_, _ = fmt.Fprintln(w)
 	r.printSummary(w)
 }
 
@@ -157,18 +157,18 @@ func (r *Report) printCheck(w io.Writer, check *CheckResult, verbose bool) {
 		prefix = style.ErrorPrefix
 	}
 
-	fmt.Fprintf(w, "%s %s: %s\n", prefix, check.Name, check.Message)
+	_, _ = fmt.Fprintf(w, "%s %s: %s\n", prefix, check.Name, check.Message)
 
 	// Print details in verbose mode or for non-OK results
 	if len(check.Details) > 0 && (verbose || check.Status != StatusOK) {
 		for _, detail := range check.Details {
-			fmt.Fprintf(w, "    %s\n", detail)
+			_, _ = fmt.Fprintf(w, "    %s\n", detail)
 		}
 	}
 
 	// Print fix hint for errors/warnings
 	if check.FixHint != "" && check.Status != StatusOK {
-		fmt.Fprintf(w, "    %s %s\n", style.ArrowPrefix, check.FixHint)
+		_, _ = fmt.Fprintf(w, "    %s %s\n", style.ArrowPrefix, check.FixHint)
 	}
 }
 
@@ -188,5 +188,5 @@ func (r *Report) printSummary(w io.Writer) {
 		parts = append(parts, style.Error.Render(fmt.Sprintf("%d errors", r.Summary.Errors)))
 	}
 
-	fmt.Fprintln(w, strings.Join(parts, ", "))
+	_, _ = fmt.Fprintln(w, strings.Join(parts, ", "))
 }