feat: enable permission-aware subagents

pi-subagents/test/fixtures/.pi/agents/memory-readwrite.md

@@ -0,0 +1,12 @@
+---
+description: "memory read-write — an agent with write tools gets a writable memory block."
+memory: user
+tools: read, write
+expect_tools_present: "read, write, edit"
+expect_prompt_contains: "persistent memory directory, Memory scope: user"
+expect_prompt_absent: "(read-only)"
+---
+A write-capable memory agent. Per the README, agents with write/edit tools get
+full read-write memory; the memory tool set is completed (edit is auto-added) and
+a writable memory block is injected into the system prompt. Scope `user` so the
+memory dir is created under the hermetic HOME, never in the repo.