docs(store): restore inline doc comments on interface methods

Restore 3 godoc comments that were accidentally dropped during
AgentStore/SessionStore interface split: GetDefault hint, CanAccess
return values, and Get vs GetOrCreate distinction.

internal/store/agent_store.go

@@ -322,7 +322,7 @@ type AgentCRUDStore interface {
 	Update(ctx context.Context, id uuid.UUID, updates map[string]any) error
 	Delete(ctx context.Context, id uuid.UUID) error
 	List(ctx context.Context, ownerID string) ([]AgentData, error)
-	GetDefault(ctx context.Context) (*AgentData, error)
+	GetDefault(ctx context.Context) (*AgentData, error) // agent with is_default=true, or first available
 }
 
 // AgentAccessStore manages agent sharing and access control.
@@ -330,7 +330,7 @@ type AgentAccessStore interface {
 	ShareAgent(ctx context.Context, agentID uuid.UUID, userID, role, grantedBy string) error
 	RevokeShare(ctx context.Context, agentID uuid.UUID, userID string) error
 	ListShares(ctx context.Context, agentID uuid.UUID) ([]AgentShareData, error)
-	CanAccess(ctx context.Context, agentID uuid.UUID, userID string) (bool, string, error)
+	CanAccess(ctx context.Context, agentID uuid.UUID, userID string) (bool, string, error) // (allowed, role, err)
 	ListAccessible(ctx context.Context, userID string) ([]AgentData, error)
 }
 

internal/store/session_store.go

@@ -89,6 +89,7 @@ type SessionListRichResult struct {
 // SessionCoreStore manages session lifecycle, messages, and history.
 type SessionCoreStore interface {
 	GetOrCreate(ctx context.Context, key string) *SessionData
+	// Get returns the session if it exists (cache or DB), nil otherwise. Never creates.
 	Get(ctx context.Context, key string) *SessionData
 	AddMessage(ctx context.Context, key string, msg providers.Message)
 	GetHistory(ctx context.Context, key string) []providers.Message