chore(mcp): point library-activate not-found hint at pad_library (TASK-1564) (#616)

The dispatcher's `library activate` not-found error referenced a
`pad_project action=library-list` action that never existed — leftover
from an earlier design draft for IDEA-1514 (the now-shipped
`pad_library` tool, PLAN-1560 / TASK-1563). The correct hint is
`pad_library action=list`.

One-line hint fix; no behavior change. Closes the remaining scope of
TASK-1564 — the onboard playbook body update shipped in PR #615
(commit 1638bdf) per Codex review. No CHANGELOG entry added: the
project has no top-level CHANGELOG.md and tracks history via git +
Pad items.

Closes TASK-1564 → completes PLAN-1560 (IDEA-1514).

internal/mcp/dispatch_http_slice4.go

@@ -571,7 +571,7 @@ func (d *HTTPHandlerDispatcher) dispatchLibraryActivate(
 	return NewErrorResult(ErrorPayload{
 		Code:    ErrNotFound,
 		Message: fmt.Sprintf("%s: %q not found in convention or playbook library", cmdKey, title),
-		Hint:    "Use `pad_project action=library-list` to enumerate available titles.",
+		Hint:    "Use `pad_library action=list` to enumerate available titles.",
 	}), nil
 }