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>

test/fixtures/context-budget.json

@@ -39,7 +39,7 @@
     "ios-sync": 2465,
     "land-and-deploy": 13585,
     "landing-report": 3269,
-    "learn": 2092,
+    "learn": 2124,
     "make-pdf": 5204,
     "modernize": 3772,
     "office-hours": 19139,