fix(tools): hint model when shell commands target DB-backed memory files

Memory files (MEMORY.md, memory/*.md) are stored in PostgreSQL, not on
disk. When the model uses exec to run cat/ls/grep/find on memory paths,
short-circuit with a hint to use memory_search or memory_get instead.
Also append [Source: database, not filesystem] to read_file and
list_files results served from the memory interceptor.

internal/tools/exec_memory_hints.go

@@ -0,0 +1,47 @@
+package tools
+
+import "strings"
+
+// memoryHintPrefixes are shell commands that read/list/search filesystem content.
+// If these target memory paths, the model should use dedicated memory tools instead.
+var memoryHintPrefixes = []string{
+	"cat ", "head ", "tail ", "less ", "more ",
+	"grep ", "rg ", "ag ",
+	"find ", "ls ", "wc ",
+}
+
+// memoryPathTokens are path fragments indicating memory file access.
+var memoryPathTokens = []string{
+	"MEMORY.md", "memory.md", "memory/",
+}
+
+// MaybeMemoryExecHint checks if a shell command targets memory files
+// (which live in the database, not on disk) and returns an LLM hint.
+// Returns empty string if no memory path detected.
+func MaybeMemoryExecHint(command string) string {
+	trimmed := strings.TrimSpace(command)
+	if trimmed == "" {
+		return ""
+	}
+
+	// Check if command uses a filesystem read/list/search binary.
+	hasReadCmd := false
+	for _, prefix := range memoryHintPrefixes {
+		if strings.HasPrefix(trimmed, prefix) {
+			hasReadCmd = true
+			break
+		}
+	}
+	if !hasReadCmd {
+		return ""
+	}
+
+	// Check if the command references a memory path.
+	for _, token := range memoryPathTokens {
+		if strings.Contains(trimmed, token) {
+			return "[HINT] Memory files are in the database, not on disk. Use memory_search or memory_get tool instead."
+		}
+	}
+
+	return ""
+}