feat(memory): define Store interface for persistent sessions

Introduce pkg/memory with a Store interface that maps 1:1 to the
current SessionManager API. Each method is an atomic operation,
eliminating the need for a separate Save() call.

This lays the groundwork for replacing JSON file-based session
storage with a transactional backend.

pkg/memory/store.go

@@ -0,0 +1,38 @@
+package memory
+
+import (
+	"context"
+
+	"github.com/sipeed/picoclaw/pkg/providers"
+)
+
+// Store defines an interface for persistent session storage.
+// Each method is an atomic operation — there is no separate Save() call.
+type Store interface {
+	// AddMessage appends a simple text message to a session.
+	AddMessage(ctx context.Context, sessionKey, role, content string) error
+
+	// AddFullMessage appends a complete message (with tool calls and tool call ID) to a session.
+	AddFullMessage(ctx context.Context, sessionKey string, msg providers.Message) error
+
+	// GetHistory returns all messages for a session in insertion order.
+	// Returns an empty slice (not nil) if the session does not exist.
+	GetHistory(ctx context.Context, sessionKey string) ([]providers.Message, error)
+
+	// GetSummary returns the conversation summary for a session.
+	// Returns an empty string if no summary exists.
+	GetSummary(ctx context.Context, sessionKey string) (string, error)
+
+	// SetSummary updates the conversation summary for a session.
+	SetSummary(ctx context.Context, sessionKey, summary string) error
+
+	// TruncateHistory removes all but the last keepLast messages from a session.
+	// If keepLast <= 0, all messages are removed.
+	TruncateHistory(ctx context.Context, sessionKey string, keepLast int) error
+
+	// SetHistory replaces all messages in a session with the provided history.
+	SetHistory(ctx context.Context, sessionKey string, history []providers.Message) error
+
+	// Close releases any resources held by the store.
+	Close() error
+}