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.go

@@ -251,24 +251,24 @@ func (g *Git) CheckConflicts(source, target string) ([]string, error) {
 		conflicts, err := g.getConflictingFiles()
 		if err == nil && len(conflicts) > 0 {
 			// Abort the test merge
-			g.AbortMerge()
+			_ = g.AbortMerge()
 			return conflicts, nil
 		}
 
 		// Check if it's a conflict error from wrapper
 		if errors.Is(mergeErr, ErrMergeConflict) {
-			g.AbortMerge()
+			_ = g.AbortMerge()
 			return conflicts, nil
 		}
 
 		// Some other merge error
-		g.AbortMerge()
+		_ = g.AbortMerge()
 		return nil, mergeErr
 	}
 
 	// Merge succeeded (no conflicts) - abort the test merge
 	// Use reset since --abort won't work on successful merge
-	g.run("reset", "--hard", "HEAD")
+	_, _ = g.run("reset", "--hard", "HEAD")
 	return nil, nil
 }