feat: enable permission-aware subagents

pi-subagents/test/fixtures/.pi/agents/tools-narrow.md

@@ -0,0 +1,9 @@
+---
+description: "tools narrowed to two built-ins; extensions omitted (=> true)."
+tools: read, grep
+expect_tools_present: "read, grep"
+expect_tools_absent: "bash, edit, write, find, ls, alpha_read, alpha_write, beta_tool"
+---
+e2e template: a plain built-in allowlist narrows to exactly the listed tools.
+extensions is omitted (defaults to true); none are discovered here, so no
+extension tools surface.