feat: enable permission-aware subagents

pi-subagents/test/fixtures/.pi/agents/lazy-ext-unselected.md

@@ -0,0 +1,10 @@
+---
+description: "A lazy extension left out of the ext: flip stays muted."
+extensions: "./ext-lazy.mjs, ./ext-alpha.mjs"
+tools: "*, ext:ext-alpha.mjs"
+expect_tools_present: "read, bash, alpha_read, alpha_write"
+expect_tools_absent: "lazy_tool"
+---
+e2e template: the mirror of lazy-ext-selected — admitting late tools must not
+mean admitting ALL late tools. ext-lazy loads and its session_start handler runs,
+but the `ext:` flip did not select it, so `lazy_tool` never becomes active.