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>

tools/file_delete.go

@@ -0,0 +1,70 @@
+// file_delete removes a saved file by its file_id. Decrements the
+// underlying blob's refcount in storage; the blob row is removed when
+// refcount hits zero.
+//
+// Why scope is checked POST-fetch (mirrors file_get): file_id is the
+// only key the caller has; we must read the row to know the scope.
+package tools
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"gitea.stevedudenhoeffer.com/steve/executus/tool"
+)
+
+type fileDeleteArgs struct {
+	FileID string `json:"file_id" description:"Opaque file ID returned by file_save or file_list."`
+}
+
+// NewFileDelete constructs the file_delete tool. storage nil → "not
+// configured" at execute time.
+func NewFileDelete(storage FileStorage) tool.Tool {
+	return tool.NewGatedTool[fileDeleteArgs](
+		"file_delete",
+		"Remove a saved file by file_id. Returns 'ok' on success or 'not_found' if no file matched.",
+		tool.Permission{
+			AuthoringRequirement: tool.RequirementAnyone,
+			OperatesOn:           tool.ScopeCaller,
+			SafeForShare:         true,
+			Categories:           []string{"storage", "write"},
+		},
+		func(ctx context.Context, inv tool.Invocation, args fileDeleteArgs) (string, error) {
+			if storage == nil {
+				return "", fmt.Errorf("file_delete: not configured")
+			}
+			if args.FileID == "" {
+				return "", fmt.Errorf("file_delete: file_id required")
+			}
+
+			// Fetch first so we can validate scope before deleting. The
+			// extra read is acceptable for a write path that's not in
+			// the hot loop, and it preserves the cross-skill /
+			// cross-user safety story.
+			meta, _, err := storage.FileGet(ctx, args.FileID)
+			if err != nil {
+				if errors.Is(err, ErrFileNotFound) {
+					return "not_found", nil
+				}
+				return "", fmt.Errorf("file_delete: %w", err)
+			}
+			if meta.SkillID != inv.SkillID {
+				return "", fmt.Errorf("file_delete: file does not belong to this skill")
+			}
+			if err := ValidateScope(inv, meta.Scope, false); err != nil {
+				return "", fmt.Errorf("file_delete: %w", err)
+			}
+
+			if err := storage.FileDelete(ctx, args.FileID); err != nil {
+				if errors.Is(err, ErrFileNotFound) {
+					// Race: row was deleted between FileGet and
+					// FileDelete. Surface as a clean miss.
+					return "not_found", nil
+				}
+				return "", fmt.Errorf("file_delete: %w", err)
+			}
+			return "ok", nil
+		},
+	)
+}