fix: emit stderr hint when no filter matches to prevent LLM confusion

When an LLM runs "snip mkdir" (no matching filter), snip silently
passes through, reinforcing the wrong mental model. Now prints:
  snip: no filter for "mkdir", passing through — you can run "mkdir" directly

This helps LLMs self-correct instead of looping on the snip prefix.

Closes #21

Author: edouard-claude

internal/engine/pipeline.go

@@ -33,8 +33,9 @@ func (p *Pipeline) Run(command string, args []string) int {
 	// Match filter
 	f := p.Registry.Match(command, subcommand, filterArgs)
 
-	// No filter found: passthrough
+	// No filter found: passthrough with hint so LLMs know snip is unnecessary
 	if f == nil {
+		fmt.Fprintf(os.Stderr, "snip: no filter for %q, passing through — you can run %q directly\n", command, command)
 		return p.Passthrough(command, args)
 	}