feat: add --update/-u flag to pull git-backed cheatpaths (#552)

Iterates over configured cheatpaths and runs git pull on each one that
is a git repository with a clean worktree. Supports SSH remotes via key
file discovery and SSH agent fallback. Works with --path filtering.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

HACKING.md

@@ -213,7 +213,7 @@ The codebase follows consistent error handling patterns:
 
 Example:
 ```go
-sheet, err := sheet.New(path, tags, false)
+s, err := sheet.New(title, cheatpath, path, tags, false)
 if err != nil {
     return fmt.Errorf("failed to load sheet: %w", err)
 }