P3: store group — kv_* + file_* tools (agent memory)

RegisterStore(reg, StoreDeps) registers the persistent-memory tools over the
host's KV and/or File backends:
- kv_get/set/list/delete (KVStorage seam)
- file_save/get/get_text/get_metadata/list/delete (FileStorage seam), plus
  file_search (FileSearcher) and create_file_url (FileTokenMinter) when wired.

Near-zero-config: Quota defaults to a generous static cap (staticQuota), the
per-value/per-file caps default, and the kv vs file groups register
independently (a host can take just one). Seams moved clean (interface-only):
kv_storage.go, quota_provider.go, file_descendant_grant.go. The default
in-memory KV/File backends come with contrib/store at P4.

Core go.sum still free of gorm/redis/discordgo/sqlite.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -60,11 +60,12 @@ CORE (majordomo + stdlib):
   compact/  context compactor (WithCompactor hook)       [P2 ✓]
   tools/    generic tool library: Register (think/now/    [P3 wip]
             cite, zero-config) + RegisterMeta (classify/
-            extract_entities/summarize); seams in
-            research_providers.go/file_storage.go;
-            in-memory budget default. End-to-end "agent
-            calls a tool" test green. Remaining: web/net/
-            store/compose groups + their backends         [P3]
+            extract_entities/summarize) + RegisterStore
+            (kv_*/file_*, default static quota); seams in
+            research_providers.go/file_storage.go/
+            kv_storage.go/quota_provider.go. End-to-end
+            "agent calls a tool" test green. Remaining:
+            web/net/compose groups + default backends     [P3]
 
 BATTERIES (opt-in siblings, each nil-safe + a default):
   persona/   Agent noun + AgentStore seam + yml loader   [P4]