fix(learn): guard the state-root lookup in Prune; re-measure the learn ceiling

Skill-Size: 2586→2707 bytes (learn/SKILL.md.tmpl)
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

docs/skills.md

@@ -910,7 +910,7 @@ gstack's own templates and for any skill folder you own.
 
 `/write-skill` is not `/skillify`: `/skillify` codifies a browser scrape into a browser-skill. Adapted from
 [nicobailon/skill-diet](https://github.com/nicobailon/skill-diet) (MIT). The first diet, /learn, cut its template from
-5,797 to 2,586 bytes.
+5,797 to 2,707 bytes.
 
 ## `/test-audit`