feat: enable permission-aware subagents

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

@@ -0,0 +1,10 @@
+---
+description: "ext: selecting the lazy extension surfaces its session_start tool."
+extensions: "./ext-lazy.mjs, ./ext-alpha.mjs"
+tools: "*, ext:ext-lazy.mjs"
+expect_tools_present: "read, bash, lazy_tool"
+expect_tools_absent: "alpha_read, alpha_write"
+---
+e2e template: the case a static allowlist can never express. `lazy_tool` does not
+exist when the session is constructed, so it cannot be listed up front — scope
+has to be re-derived once the extension registers it.