chore(vendor): import pi 0.85.1

Imported as a linear vendor commit so forge rebases do not replay the upstream root tree.

git-subtree-dir: third_party/pi

git-subtree-split: d981de1229ef899957bbe968bc8dcda02a21f477

third_party/pi/packages/coding-agent/test/fixtures/skills/unknown-field/SKILL.md

@@ -0,0 +1,10 @@
+---
+name: unknown-field
+description: A skill with an unknown frontmatter field.
+author: someone
+version: 1.0
+---
+
+# Unknown Field
+
+This skill has non-standard frontmatter fields.