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/cmd/init.go

@@ -64,7 +64,7 @@ func runInit(cmd *cobra.Command, args []string) error {
 		// Create .gitkeep to ensure directory is tracked if needed
 		gitkeep := filepath.Join(dirPath, ".gitkeep")
 		if _, err := os.Stat(gitkeep); os.IsNotExist(err) {
-			os.WriteFile(gitkeep, []byte(""), 0644)
+			_ = os.WriteFile(gitkeep, []byte(""), 0644)
 		}
 
 		fmt.Printf("   ✓ Created %s/\n", dir)