feat: enable permission-aware subagents

pi-subagents/test/fixtures/.pi/agents/minimal.md

@@ -0,0 +1,9 @@
+---
+description: "Minimal agent — only description and body; every other field omitted."
+expect_tools_present: "read, bash, edit, write, grep, find, ls"
+expect_tools_absent: "alpha_read, alpha_write, beta_tool"
+---
+A minimal agent. Per the README defaults: omitted `tools` => all 7 built-ins;
+omitted `extensions` => true (all *discovered* extensions load — none exist in
+this hermetic fixture, so no extension tools surface). expect_* are test-harness
+annotations and are ignored by the agent loader.